3.x: Remove redundant addition of JMH to classpath

[benjamintboyle]

Remove manual addition of JMH classpath in build file. Gradle adds classpath
for all sourceSets to Eclipse by default as fixed in Gradle 6.8 by PR
14534. The JMH sourceSet is added by the JMH Plugin.

Tested by running 'eclipse' task in Eclipse 2021-03 (4.19.0). Then refreshing,
and JMH still on classpath and viewable in Eclipse. Then removed project from
Eclipse, ran 'cleanEclipse', and re-imported project to Eclipse. All successful.

[codecov]

Codecov Report

Merging #7257 (d86131f) into 3.x (06294de) will decrease coverage by 0.05%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                3.x    #7257      +/-   ##
============================================
- Coverage     99.57%   99.51%   -0.06%     
+ Complexity     6758     6756       -2     
============================================
  Files           747      747              
  Lines         47391    47391              
  Branches       6382     6382              
============================================
- Hits          47190    47163      -27     
- Misses           91       99       +8     
- Partials        110      129      +19     

Impacted Files	Coverage Δ	Complexity Δ
...l/operators/observable/ObservableFlatMapMaybe.java	89.43% <0.00%> (-7.75%)	2.00% <0.00%> (ø%)
...tivex/rxjava3/internal/jdk8/ParallelCollector.java	93.57% <0.00%> (-4.59%)	2.00% <0.00%> (ø%)
...x/rxjava3/internal/operators/single/SingleAmb.java	96.61% <0.00%> (-3.39%)	9.00% <0.00%> (-1.00%)
...nternal/operators/observable/ObservableWindow.java	98.21% <0.00%> (-1.79%)	3.00% <0.00%> (ø%)
...ators/observable/ObservableFlatMapCompletable.java	98.52% <0.00%> (-1.48%)	2.00% <0.00%> (ø%)
...ternal/operators/completable/CompletableMerge.java	97.29% <0.00%> (-1.36%)	2.00% <0.00%> (ø%)
...operators/flowable/FlowableFlatMapCompletable.java	98.71% <0.00%> (-1.29%)	2.00% <0.00%> (ø%)
...rnal/operators/flowable/FlowableFlatMapSingle.java	95.34% <0.00%> (-1.17%)	2.00% <0.00%> (ø%)
...ternal/operators/observable/ObservableGroupBy.java	98.90% <0.00%> (-1.10%)	2.00% <0.00%> (ø%)
...operators/observable/ObservableMergeWithMaybe.java	99.09% <0.00%> (-0.91%)	2.00% <0.00%> (ø%)
... and 12 more

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