2.x: Adjust JavaDocs dl/dd entry stylesheet

[akarnokd]

The default JavaDocs stylesheet uses monospaced for the Scheduler/Backpressure/etc definitions list entries which practically makes any {@code } marking practically invisible. This PR set up a stylesheet with that specific style definition reverted to default. There was an unused stylesheet.css in the project from long before.

Before:

After:

[codecov]

Codecov Report

Merging #6070 into 2.x will decrease coverage by 0.08%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #6070      +/-   ##
============================================
- Coverage     98.31%   98.23%   -0.09%     
+ Complexity     6194     6191       -3     
============================================
  Files           666      666              
  Lines         44806    44806              
  Branches       6206     6206              
============================================
- Hits          44053    44016      -37     
- Misses          229      245      +16     
- Partials        524      545      +21

Impacted Files	Coverage Δ	Complexity Δ
...l/operators/observable/ObservableFlatMapMaybe.java	90.19% <0%> (-6.54%)	2% <0%> (ø)
...nternal/operators/parallel/ParallelReduceFull.java	91.08% <0%> (-3.97%)	2% <0%> (ø)
...a/io/reactivex/internal/util/QueueDrainHelper.java	97.22% <0%> (-2.78%)	56% <0%> (-2%)
...activex/internal/observers/QueueDrainObserver.java	97.43% <0%> (-2.57%)	21% <0%> (-1%)
...java/io/reactivex/processors/PublishProcessor.java	98.19% <0%> (-1.81%)	42% <0%> (-1%)
...x/internal/operators/flowable/FlowablePublish.java	97.44% <0%> (-1.71%)	11% <0%> (ø)
...internal/operators/flowable/FlowableFromArray.java	94.91% <0%> (-1.7%)	3% <0%> (ø)
...perators/mixed/ObservableConcatMapCompletable.java	98.49% <0%> (-1.51%)	3% <0%> (ø)
...rnal/operators/flowable/FlowableFlatMapSingle.java	95.65% <0%> (-1.09%)	2% <0%> (ø)
...perators/mixed/ObservableSwitchMapCompletable.java	98.94% <0%> (-1.06%)	3% <0%> (ø)
... and 18 more

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