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

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.

Locate the Java templates that generate ApiClient and the classes in the apiPackage, then inspect createDefaultAdapter() and the RxJava imports. Generate a client with useRxJava set to false and verify that the RxJava adapter, Observable, and Completable imports and dependencies are no longer required.