3.x: Remove fromFuture(..., Scheduler) overloads

[akarnokd]

Those overloads were only applying subscribeOn with no other benefits.

Resolves: #6811

[codecov]

Codecov Report

Merging #6814 into 3.x will increase coverage by 0.02%.
The diff coverage is n/a.

@@ Coverage Diff @@ ## 3.x #6814 +/- ## ============================================ + Coverage 98.26% 98.28% +0.02%  + Complexity 6385 6378 -7  ============================================ Files 715 715 Lines 46498 46486 -12 Branches 6371 6371 ============================================ - Hits 45689 45687 -2  + Misses 284 283 -1  + Partials 525 516 -9

Impacted Files	Coverage Δ	Complexity Δ
...rc/main/java/io/reactivex/rxjava3/core/Single.java	100% <ø> (ø)	151 <0> (-2)	⬇️
.../main/java/io/reactivex/rxjava3/core/Flowable.java	100% <ø> (ø)	575 <0> (-2)	⬇️
...ain/java/io/reactivex/rxjava3/core/Observable.java	100% <ø> (ø)	551 <0> (-2)	⬇️
.../operators/observable/ObservableFlatMapSingle.java	88.88% <0%> (-6.35%)	2% <0%> (ø)
...eactivex/rxjava3/processors/BehaviorProcessor.java	96.55% <0%> (-2.47%)	50% <0%> (ø)
...ivex/rxjava3/subscribers/SerializedSubscriber.java	98.86% <0%> (-1.14%)	26% <0%> (-1%)
...a3/internal/operators/flowable/FlowableCreate.java	96.44% <0%> (-0.98%)	6% <0%> (ø)
...rxjava3/internal/schedulers/ExecutorScheduler.java	95.89% <0%> (-0.92%)	10% <0%> (ø)
...3/internal/operators/flowable/FlowableGroupBy.java	95.59% <0%> (-0.63%)	3% <0%> (ø)
.../reactivex/rxjava3/observers/BaseTestConsumer.java	96.68% <0%> (-0.56%)	68% <0%> (-1%)
... and 13 more

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