[BUG] [Java] RxJava2CallAdapterFactory should not be used & io.reactivex.Observable / io.reactivex.Completable not imported when useRxJava2: 'false'

[vorburger]

When using useRxJava2: 'false', then the com.squareup.retrofit2:adapter-rxjava2 dependency (and due to & through it transitively its own dependency org.reactivestreams:reactive-streams) are still required - even though they should not be used in the generated code with that configuration.

The reason for that is that the generated ApiClient has an import retrofit2.adapter.rxjava2.RxJava2CallAdapterFactory because createDefaultAdapter() does a .addCallAdapterFactory(RxJava2CallAdapterFactory.create()) - which, of course, isn't actually needed when using useRxJava2: 'false'. Similarly, all classes generated into the apiPackage folder always have an import io.reactivex.Observable; and a import io.reactivex.Completable; even though, again, they are not used anywhere with useRxJava2: 'false'.

The templates could thus be improved to skip those lines for users who don't need Rx.

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