2.x: Fix javadocs & imports

[akarnokd]

• Fix some errors in the Javadoc links.
• Remove unused imports.
• Add the ParallelFlowable page to the javadoc cleanup routine.

[codecov]

Codecov Report

Merging #6504 into 2.x will decrease coverage by <.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #6504      +/-   ##
============================================
- Coverage     98.27%   98.27%   -0.01%     
+ Complexity     6291     6290       -1     
============================================
  Files           675      675              
  Lines         45163    45163              
  Branches       6244     6244              
============================================
- Hits          44386    44382       -4     
  Misses          241      241              
- Partials        536      540       +4

Impacted Files	Coverage Δ	Complexity Δ
.../main/java/io/reactivex/schedulers/Schedulers.java	100% <ø> (ø)	11 <0> (ø)	⬇️
...nternal/operators/parallel/ParallelReduceFull.java	91.08% <0%> (-3.97%)	2% <0%> (ø)
...ternal/operators/observable/ObservablePublish.java	93.8% <0%> (-3.54%)	10% <0%> (ø)
...ernal/operators/flowable/FlowableFromIterable.java	94.11% <0%> (-3.21%)	5% <0%> (ø)
...va/io/reactivex/internal/queue/SpscArrayQueue.java	97.61% <0%> (-2.39%)	22% <0%> (-1%)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	93.23% <0%> (-1.94%)	2% <0%> (ø)
.../io/reactivex/disposables/CompositeDisposable.java	98.14% <0%> (-1.86%)	39% <0%> (-1%)
...tivex/internal/operators/single/SingleTimeout.java	98.33% <0%> (-1.67%)	2% <0%> (ø)
...l/operators/observable/ObservableFlatMapMaybe.java	92.15% <0%> (-1.31%)	2% <0%> (ø)
...ternal/operators/completable/CompletableMerge.java	97.61% <0%> (-1.2%)	2% <0%> (ø)
... and 22 more

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