fix(typescript-angular): fix model filename including `modelNamePrefix`

Created by: terencehonles

This change removes the modelNamePrefix from the API import filename to be consistent with the processing in the rest of the class.

To test: run openapi-generator-cli generate -i api.yml -g typescript-angular -o gen/api --additional-properties=modelNamePrefix=I

In the original code you'll see:

// model.ts
// <header>
import { IOtherModel } from './other-model';
...

// service.ts
// <header>
// <external-imports>

// @ts-ignore                                                                  
import { IModel } from '../model/i-model'; /** There is no file with name "i-model.ts" */
...

with the changes the first file will remain the same but the second will be:

// service.ts
// <header>
// <external-imports>

// @ts-ignore                                                                  
import { IModel } from '../model/model'; /** This is correct 👍  */
...

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