3.x: remove buffer/window with supplier & onExceptionResumeNext

[akarnokd]

This PR removes 3 kinds of operators that had no noticeable use bases or use cases on their own in the past years.

• buffer(Supplier [, Supplier])
• window(Supplier [, int])
• onExceptionResumeNext(<source>)

Resolves #6554
Resolves #6555

[codecov]

Codecov Report

Merging #6564 into 3.x will increase coverage by <.01%.
The diff coverage is 100%.

@@ Coverage Diff @@ ## 3.x #6564 +/- ## ============================================ + Coverage 98.19% 98.19% +<.01%  + Complexity 6199 6181 -18  ============================================ Files 682 678 -4 Lines 45461 44886 -575 Branches 6290 6212 -78 ============================================ - Hits 44640 44076 -564  - Misses 268 274 +6  + Partials 553 536 -17

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Flowable.java	100% <100%> (ø)	565 <0> (-5)	⬇️
...al/operators/observable/ObservableOnErrorNext.java	100% <100%> (ø)	2 <1> (ø)	⬇️
src/main/java/io/reactivex/Observable.java	100% <100%> (ø)	540 <0> (-5)	⬇️
...ternal/operators/flowable/FlowableOnErrorNext.java	100% <100%> (ø)	2 <1> (ø)	⬇️
.../operators/observable/ObservableFlatMapSingle.java	88.8% <0%> (-5.23%)	2% <0%> (ø)
...tivex/internal/observers/FutureSingleObserver.java	94.33% <0%> (-3.78%)	24% <0%> (-1%)
...perators/single/SingleFlatMapIterableFlowable.java	95.83% <0%> (-2.5%)	2% <0%> (ø)
...ava/io/reactivex/processors/BehaviorProcessor.java	96.58% <0%> (-2.44%)	51% <0%> (ø)
...ternal/operators/completable/CompletableMerge.java	96.42% <0%> (-2.39%)	2% <0%> (ø)
...nternal/operators/parallel/ParallelSortedJoin.java	92.7% <0%> (-2.19%)	2% <0%> (ø)
... and 23 more

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