2.x: improve wording and links in package-infos + remove unused imports

[akarnokd]

This PR is to improve the wording of a few package-info.java JavaDocs and removes two unused imports.

[codecov]

Codecov Report

Merging #5746 into 2.x will decrease coverage by 0.04%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #5746      +/-   ##
============================================
- Coverage     96.25%   96.21%   -0.05%     
- Complexity     5833     5836       +3     
============================================
  Files           634      634              
  Lines         41644    41644              
  Branches       5768     5768              
============================================
- Hits          40086    40069      -17     
- Misses          625      638      +13     
- Partials        933      937       +4

Impacted Files	Coverage Δ	Complexity Δ
.../internal/operators/single/SingleFromCallable.java	100% <ø> (ø)	5 <0> (ø)	⬇️
...nternal/operators/observable/ObservableCreate.java	79.48% <0%> (-17.95%)	2% <0%> (ø)
.../operators/flowable/FlowableBlockingSubscribe.java	91.89% <0%> (-5.41%)	9% <0%> (-1%)
.../io/reactivex/internal/functions/ObjectHelper.java	94.73% <0%> (-5.27%)	20% <0%> (-1%)
.../operators/observable/ObservableFlatMapSingle.java	91.04% <0%> (-2.99%)	2% <0%> (ø)
...tivex/internal/schedulers/TrampolineScheduler.java	92.3% <0%> (-2.57%)	6% <0%> (ø)
.../io/reactivex/disposables/CompositeDisposable.java	97.24% <0%> (-1.84%)	39% <0%> (-1%)
...vex/internal/operators/flowable/FlowableCache.java	93.95% <0%> (-1.35%)	7% <0%> (ø)
...x/internal/operators/flowable/FlowablePublish.java	94.27% <0%> (-1.33%)	11% <0%> (+1%)
...l/operators/observable/ObservableFlatMapMaybe.java	90.84% <0%> (-1.31%)	2% <0%> (ø)
... and 20 more

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