Remove mentions of Main thread from Schedulers.single() javadoc.

[artem-zinnatullin]

To avoid confusion.

One could think that single() can be used to execute code on "Main" thread.

[artem-zinnatullin]

nit: I don't really understand use of "Uses" here… so if you have some suggestions, I could update this as well

[codecov]

Codecov Report

Merging #5706 into 2.x will increase coverage by 0.06%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #5706      +/-   ##
============================================
+ Coverage     96.26%   96.32%   +0.06%     
- Complexity     5821     5825       +4     
============================================
  Files           634      634              
  Lines         41604    41604              
  Branches       5761     5761              
============================================
+ Hits          40049    40077      +28     
+ Misses          618      602      -16     
+ Partials        937      925      -12

Impacted Files	Coverage Δ	Complexity Δ
.../main/java/io/reactivex/schedulers/Schedulers.java	100% <ø> (ø)	10 <0> (ø)	⬇️
...in/java/io/reactivex/subjects/BehaviorSubject.java	86.97% <0%> (-4.69%)	57% <0%> (ø)
...nternal/operators/observable/ObservableWindow.java	98% <0%> (-2%)	3% <0%> (ø)
...ex/internal/subscribers/InnerQueuedSubscriber.java	96.07% <0%> (-1.97%)	18% <0%> (-1%)
.../operators/observable/ObservableFlatMapSingle.java	94.77% <0%> (-1.5%)	2% <0%> (ø)
...a/io/reactivex/internal/util/QueueDrainHelper.java	60.28% <0%> (-1.42%)	32% <0%> (-1%)
...vex/internal/operators/flowable/FlowableCache.java	92.61% <0%> (-1.35%)	7% <0%> (ø)
...nal/operators/flowable/FlowableConcatMapEager.java	97.1% <0%> (-1.16%)	2% <0%> (ø)
...ivex/internal/operators/maybe/MaybeMergeArray.java	96.62% <0%> (-1.13%)	6% <0%> (ø)
...tors/observable/ObservableSequenceEqualSingle.java	98.29% <0%> (-0.86%)	3% <0%> (ø)
... and 27 more

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