2.x: fix timed exact buffer calling cancel unnecessarily

[akarnokd]

This PR removes the unnnecessary cancel/dispose call from the exact-boundary timed buffer() operators in Observable and Flowable when the upstream completes normally.

Originally appeared in a StackOverflow question.

[codecov]

Codecov Report

Merging #5761 into 2.x will decrease coverage by <.01%.
The diff coverage is 50%.

@@             Coverage Diff              @@
##                2.x    #5761      +/-   ##
============================================
- Coverage     96.24%   96.24%   -0.01%     
- Complexity     5838     5839       +1     
============================================
  Files           634      634              
  Lines         41653    41656       +3     
  Branches       5769     5772       +3     
============================================
+ Hits          40090    40091       +1     
+ Misses          624      621       -3     
- Partials        939      944       +5

Impacted Files	Coverage Δ	Complexity Δ
...al/operators/observable/ObservableBufferTimed.java	92.19% <100%> (ø)	5 <0> (ø)	⬇️
...ternal/operators/flowable/FlowableBufferTimed.java	90.1% <100%> (-0.74%)	5 <0> (ø)
...a/io/reactivex/internal/util/QueueDrainHelper.java	60.41% <33.33%> (-1.29%)	34 <0> (+1)
...ternal/operators/observable/ObservablePublish.java	92.1% <0%> (-4.39%)	10% <0%> (-1%)
...erators/completable/CompletableConcatIterable.java	95.91% <0%> (-4.09%)	2% <0%> (ø)
...l/operators/observable/ObservableFlatMapMaybe.java	84.96% <0%> (-3.93%)	2% <0%> (ø)
.../internal/operators/flowable/FlowableInterval.java	94.44% <0%> (-2.78%)	3% <0%> (ø)
.../operators/observable/ObservableFlatMapSingle.java	91.79% <0%> (-2.24%)	2% <0%> (ø)
...nternal/operators/parallel/ParallelReduceFull.java	93.13% <0%> (-1.97%)	2% <0%> (ø)
...rnal/subscriptions/DeferredScalarSubscription.java	98.46% <0%> (-1.54%)	28% <0%> (-1%)
... and 22 more

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[vanniktech]

Huh this totally slipped on my side.