Remove unnecessary generic type parameters for delaySubscription methods in Single

[plackemacher]

I was running into an issue with using the delaySubscription(long time, TimeUnit unit) while testing a Single and I saw that the U generic type parameter was there unnecessarily. I believe this is only a Kotlin specific issue as I tried playing around in Java and there was no inference error upon compilation. To get it to compile for Kotlin, I would need to specify the generic type parameter and it didn't matter what I set it to as Any was a valid option: .delaySubscription<Any>(5, TimeUnit.SECONDS). I also went through Flowable, Observable, and Maybe and saw that the generics were missing for the time-specific delaySubscription methods so I believe they should be removed from Single as well to help keep things consistent and remove this compilation error for future Kotlin users.

[codecov]

Codecov Report

Merging #5511 into 2.x will increase coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##               2.x    #5511      +/-   ##
===========================================
+ Coverage     96.1%   96.11%   +0.01%     
- Complexity    5807     5808       +1     
===========================================
  Files          631      631              
  Lines        41285    41285              
  Branches      5732     5732              
===========================================
+ Hits         39676    39681       +5     
+ Misses         638      629       -9     
- Partials       971      975       +4

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Single.java	100% <ø> (ø)	132 <0> (ø)	⬇️
.../operators/observable/ObservableFlatMapSingle.java	88.8% <0%> (-5.98%)	2% <0%> (ø)
...ternal/operators/flowable/FlowableSampleTimed.java	91.17% <0%> (-5.89%)	3% <0%> (ø)
.../operators/flowable/FlowableBlockingSubscribe.java	91.66% <0%> (-5.56%)	9% <0%> (-1%)
...activex/internal/operators/single/SingleUsing.java	95.18% <0%> (-4.82%)	4% <0%> (ø)
...tivex/internal/observers/FutureSingleObserver.java	94.33% <0%> (-3.78%)	24% <0%> (-1%)
...internal/operators/completable/CompletableAmb.java	94.91% <0%> (-3.39%)	10% <0%> (-1%)
...activex/internal/operators/single/SingleCache.java	97.05% <0%> (-2.95%)	23% <0%> (-1%)
...ternal/operators/completable/CompletableUsing.java	97.61% <0%> (-2.39%)	4% <0%> (ø)
...operators/observable/ObservableConcatMapEager.java	97.26% <0%> (-2.19%)	2% <0%> (ø)
... and 34 more

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[akarnokd]

Thanks. The JLS seems to consider such change as binary compatible.

/cc @JakeWharton