This issue collects the API changes proposed for 3.x: unused operators and overloads, mistakes in signatures, etc.

• Remove: Flowable.subscribe(/* 4 args */) with Consumer<? super Subscription> -> 3.x: Various small API changes and removals #6517
• Remove: Observable.subscribe(/* 4 args */) with Consumer<? super Disposable> -> 3.x: Various small API changes and removals #6517
• Change: Completable.blockingGet() signature -> 3.x: Various small API changes and removals #6517
• Change: Maybe.defaultIfEmpty to return Single. -> 3.x: Various small API changes and removals #6517
• Change: X.to() to use dedicated non-generic functional type which then can be implemented by a single class for multiple base types. (3.x: Proposal: ObservableConverter interface and friends #5654) -> 3.x: Merge as() into to() #6514
• Remove: BehaviorSubject.getValues() -> 3.x: remove getValues() from some subjects/processors #6516
• Remove: BehaviorProcessor.getValues() -> 3.x: remove getValues() from some subjects/processors #6516
• Change: Single.toCompletable to Single.ignoreElement to be consistent with the other types. -> 3.x: Various small API changes and removals #6517
• Remove: AsyncSubject.getValues() -> 3.x: remove getValues() from some subjects/processors #6516
• Remove: AsyncProcessor.getValues() -> 3.x: remove getValues() from some subjects/processors #6516