Remove @NonNull annotations for functions with void return type

[pahakorolev]

Minor code readability improvement, which does not make you wonder on @nonnull void type.

[codecov]

Codecov Report

Merging #7425 (d184e40) into 3.x (23c9f4d) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                3.x    #7425      +/-   ##
============================================
- Coverage     99.55%   99.55%   -0.01%     
  Complexity     6781     6781              
============================================
  Files           751      751              
  Lines         47482    47482              
  Branches       6378     6378              
============================================
- Hits          47272    47271       -1     
- Misses           92       96       +4     
+ Partials        118      115       -3     

Impacted Files	Coverage Δ
...ain/java/io/reactivex/rxjava3/core/Observable.java	100.00% <ø> (ø)
...tivex/rxjava3/internal/jdk8/ParallelCollector.java	91.74% <0.00%> (-6.43%)	⬇️
.../operators/flowable/FlowableBlockingSubscribe.java	93.02% <0.00%> (-4.66%)	⬇️
...a/io/reactivex/rxjava3/subjects/SingleSubject.java	97.53% <0.00%> (-2.47%)	⬇️
...ernal/operators/flowable/FlowableFlatMapMaybe.java	95.85% <0.00%> (-0.52%)	⬇️
...a/io/reactivex/rxjava3/subjects/ReplaySubject.java	98.75% <0.00%> (-0.21%)	⬇️
...rnal/operators/flowable/FlowableFlatMapSingle.java	96.51% <0.00%> (ø)
...3/internal/operators/flowable/FlowableGroupBy.java	85.02% <0.00%> (+0.29%)	⬆️
.../operators/observable/ObservableCombineLatest.java	100.00% <0.00%> (+0.61%)	⬆️
...l/operators/observable/ObservableFlatMapMaybe.java	97.88% <0.00%> (+0.70%)	⬆️
... and 4 more

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