Removed default options from some Babel plugins

Created by: adityavohra7

This PR removes default config options from some Babel plugins in babel-preset-react-app:

• Removed the regenerator option for babel-plugin-transform-runtime (defaults to true).
• Removed the useBuiltIns option for babel-preset-env (defaults to false).

I tested these changes by running npm test in the create-react-app root dir (tests passed), and transpiling a simple file (verified that the output was the same before and after these changes).

Here's the file I used:

// src/foo.js

import 'babel-polyfill';

function *foo() {
    yield 42;
}

BEFORE

$ NODE_ENV=production ./node_modules/.bin/babel src/foo.js yields:

import _regeneratorRuntime from 'babel-runtime/regenerator';

var _marked = [foo].map(_regeneratorRuntime.mark);

import 'babel-polyfill';

function foo() {
    return _regeneratorRuntime.wrap(function foo$(_context) {
        while (1) {
            switch (_context.prev = _context.next) {
                case 0:
                    _context.next = 2;
                    return 42;

                case 2:
                case 'end':
                    return _context.stop();
            }
        }
    }, _marked[0], this);
}

$ NODE_ENV=development ./node_modules/.bin/babel src/foo.js yields:

import _regeneratorRuntime from 'babel-runtime/regenerator';

var _marked = [foo].map(_regeneratorRuntime.mark);

import 'babel-polyfill';

function foo() {
    return _regeneratorRuntime.wrap(function foo$(_context) {
        while (1) {
            switch (_context.prev = _context.next) {
                case 0:
                    _context.next = 2;
                    return 42;

                case 2:
                case 'end':
                    return _context.stop();
            }
        }
    }, _marked[0], this);
}

$ NODE_ENV=test ./node_modules/.bin/babel src/foo.js yields:

'use strict';

var _regenerator = require('babel-runtime/regenerator');

var _regenerator2 = _interopRequireDefault(_regenerator);

require('babel-polyfill');

function _interopRequireDefault(obj) { return obj && obj.__esModule ? obj : { default: obj }; }

var _marked = [foo].map(_regenerator2.default.mark);

function foo() {
    return _regenerator2.default.wrap(function foo$(_context) {
        while (1) {
            switch (_context.prev = _context.next) {
                case 0:
                    _context.next = 2;
                    return 42;

                case 2:
                case 'end':
                    return _context.stop();
            }
        }
    }, _marked[0], this);
}

AFTER

$ NODE_ENV=production ./node_modules/.bin/babel src/foo.js yields:

import _regeneratorRuntime from 'babel-runtime/regenerator';

var _marked = [foo].map(_regeneratorRuntime.mark);

import 'babel-polyfill';

function foo() {
    return _regeneratorRuntime.wrap(function foo$(_context) {
        while (1) {
            switch (_context.prev = _context.next) {
                case 0:
                    _context.next = 2;
                    return 42;

                case 2:
                case 'end':
                    return _context.stop();
            }
        }
    }, _marked[0], this);
}

$ NODE_ENV=development ./node_modules/.bin/babel src/foo.js yields:

import _regeneratorRuntime from 'babel-runtime/regenerator';

var _marked = [foo].map(_regeneratorRuntime.mark);

import 'babel-polyfill';

function foo() {
    return _regeneratorRuntime.wrap(function foo$(_context) {
        while (1) {
            switch (_context.prev = _context.next) {
                case 0:
                    _context.next = 2;
                    return 42;

                case 2:
                case 'end':
                    return _context.stop();
            }
        }
    }, _marked[0], this);
}

$ NODE_ENV=test ./node_modules/.bin/babel src/foo.js yields:

'use strict';

var _regenerator = require('babel-runtime/regenerator');

var _regenerator2 = _interopRequireDefault(_regenerator);

require('babel-polyfill');

function _interopRequireDefault(obj) { return obj && obj.__esModule ? obj : { default: obj }; }

var _marked = [foo].map(_regenerator2.default.mark);

function foo() {
    return _regenerator2.default.wrap(function foo$(_context) {
        while (1) {
            switch (_context.prev = _context.next) {
                case 0:
                    _context.next = 2;
                    return 42;

                case 2:
                case 'end':
                    return _context.stop();
            }
        }
    }, _marked[0], this);
}

Outputs for all envs seem to be the same.