Remove custom Gradle garbage collection tasks

[benjamintboyle]

Garbage collection between Gradle tasks should no longer be needed now
that tasks are run on Github servers. Also it was only freeing up a few dozen MB.

[codecov]

Codecov Report

Merging #7225 (02bf19a) into 3.x (5a678bb) will decrease coverage by 0.04%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                3.x    #7225      +/-   ##
============================================
- Coverage     99.55%   99.51%   -0.05%     
- Complexity     6756     6757       +1     
============================================
  Files           747      747              
  Lines         47391    47391              
  Branches       6382     6382              
============================================
- Hits          47178    47159      -19     
- Misses           94      101       +7     
- Partials        119      131      +12     

Impacted Files	Coverage Δ	Complexity Δ
...l/operators/observable/ObservableFlatMapMaybe.java	92.25% <0.00%> (-4.93%)	2.00% <0.00%> (ø%)
...rnal/operators/flowable/FlowableFlatMapSingle.java	94.18% <0.00%> (-2.33%)	2.00% <0.00%> (ø%)
...vable/ObservableFlatMapCompletableCompletable.java	98.48% <0.00%> (-1.52%)	3.00% <0.00%> (ø%)
...ators/observable/ObservableFlatMapCompletable.java	98.52% <0.00%> (-1.48%)	2.00% <0.00%> (ø%)
...va3/internal/operators/parallel/ParallelRunOn.java	98.53% <0.00%> (-1.47%)	8.00% <0.00%> (ø%)
...internal/operators/flowable/FlowableSwitchMap.java	98.58% <0.00%> (-1.42%)	3.00% <0.00%> (ø%)
...a3/internal/operators/flowable/FlowableReplay.java	98.71% <0.00%> (-1.08%)	21.00% <0.00%> (ø%)
...perators/single/SingleFlatMapIterableFlowable.java	99.16% <0.00%> (-0.84%)	2.00% <0.00%> (ø%)
...va3/internal/operators/flowable/FlowableCache.java	99.24% <0.00%> (-0.76%)	38.00% <0.00%> (-1.00%)
...ternal/operators/observable/ObservableGroupBy.java	99.45% <0.00%> (-0.55%)	2.00% <0.00%> (ø%)
... and 7 more

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