3.x: Various small API changes and removals

[akarnokd]

• Maybe.defaultIfEmpty() returns Single.
• Removal of Maybe.toSingle(T).
• Removal of Flowable.subscribe(4 args)
• Removal of Observable.subscribe(4 args)
• Removal of Single.toCompletable()
• Removal of Completable.blockingGet()
• Removal of Flowable.subscribe(/* 4 args */)
• Removal of Observable.subscribe(/* 4 args */)

[codecov]

Codecov Report

Merging #6517 into 3.x will increase coverage by 0.07%.
The diff coverage is 100%.

@@ Coverage Diff @@ ## 3.x #6517 +/- ## ============================================ + Coverage 98.12% 98.19% +0.07%  + Complexity 6250 6245 -5  ============================================ Files 675 675 Lines 45083 45051 -32 Branches 6226 6225 -1 ============================================ + Hits 44236 44237 +1  + Misses 293 274 -19  + Partials 554 540 -14

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Completable.java	100% <ø> (ø)	117 <0> (-2)	⬇️
src/main/java/io/reactivex/Single.java	100% <ø> (ø)	147 <0> (-1)	⬇️
...ivex/internal/observers/BlockingMultiObserver.java	100% <ø> (ø)	19 <0> (-5)	⬇️
src/main/java/io/reactivex/Flowable.java	100% <100%> (ø)	565 <3> (-1)	⬇️
src/main/java/io/reactivex/Observable.java	100% <100%> (ø)	540 <3> (-1)	⬇️
src/main/java/io/reactivex/Maybe.java	100% <100%> (ø)	171 <0> (-1)	⬇️
...rnal/operators/observable/ObservableSwitchMap.java	93.61% <0%> (-1.07%)	3% <0%> (ø)
.../io/reactivex/internal/schedulers/IoScheduler.java	89.36% <0%> (-1.07%)	9% <0%> (ø)
...operators/observable/ObservableMergeWithMaybe.java	99.1% <0%> (-0.9%)	2% <0%> (ø)
.../operators/mixed/FlowableConcatMapCompletable.java	99.14% <0%> (-0.86%)	2% <0%> (ø)
... and 23 more

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