3.x: remove getValues() from some subjects/processors

[akarnokd]

The getValue() and getValues(T[]) methods were a remnant from a time where Subject and FlowableProcessor was unifying all state peeking methods for every kind of subject/processor. These have been marked as @Deprecated in 2.x and are now removed from 3.x. They can be trivially replaced with getValue() if necessary, for example:

Object value = subject.getValue();
if (value == null) {
   return new Object[1];
}
return new Object[] { value };

Related: #5622

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (3.x@cb03724). Click here to learn what that means.
The diff coverage is n/a.

@@          Coverage Diff           @@
##             3.x    #6516   +/-   ##
======================================
  Coverage       ?   98.19%           
  Complexity     ?     6259           
======================================
  Files          ?      675           
  Lines          ?    45107           
  Branches       ?     6226           
======================================
  Hits           ?    44293           
  Misses         ?      280           
  Partials       ?      534

Impacted Files	Coverage Δ	Complexity Δ
.../main/java/io/reactivex/subjects/AsyncSubject.java	98.93% <ø> (ø)	45 <0> (?)
...n/java/io/reactivex/processors/AsyncProcessor.java	98.95% <ø> (ø)	45 <0> (?)
...ava/io/reactivex/processors/BehaviorProcessor.java	95.6% <ø> (ø)	50 <0> (?)
...in/java/io/reactivex/subjects/BehaviorSubject.java	98.8% <ø> (ø)	45 <0> (?)

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