Make typescript-fetch code compatiable with nodejs environment

Created by: duxiaofeng-github

window should not be used in typescript-fetch because window does not exist under node js environment.

According to https://github.com/OpenAPITools/openapi-generator/pull/3326, fetch is globally defined in both browser and nodejs.

So we just need to simply use fetch.

Also I found out why the fetch function need to bind window again in the old code.

Because when the fetch function passed by the fetchApi getter, the fetch function will lose its context(window) and cause an error 'Fetch TypeError: Failed to execute 'fetch' on 'Window': Illegal invocation'.

So, DO NOT pass the fetch function by the fetchApi getter, call fetch directly wherever it needs to be executed.

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