2.x: disable codecov-checks on PRs

[akarnokd]

Codecov.io seems to be messing around quite a bit lately. A day after the integration, new checks appeared at every PR that fail because of small irrelevant fluctuations in the coverage itself and they fail the whole PR even though the tests pass.

This PR seeks to disable these checks.

[codecov]

Codecov Report

Merging #5057 into 2.x will increase coverage by 0.04%.

@@             Coverage Diff              @@
##                2.x    #5057      +/-   ##
============================================
+ Coverage     95.57%   95.62%   +0.04%     
+ Complexity     5542     5539       -3     
============================================
  Files           612      612              
  Lines         39576    39576              
  Branches       5553     5553              
============================================
+ Hits          37824    37843      +19     
+ Misses          764      750      -14     
+ Partials        988      983       -5

Impacted Files	Coverage Δ	Complexity Δ
...rnal/operators/completable/CompletableTimeout.java	92.3% <ø> (-5.13%)	2% <ø> (ø)
...rnal/subscriptions/DeferredScalarSubscription.java	93.84% <ø> (-4.62%)	27% <ø> (-1%)
.../operators/observable/ObservableFlatMapSingle.java	92.53% <ø> (-3.74%)	2% <ø> (ø)
...vex/internal/operators/flowable/FlowableTimer.java	92.85% <ø> (-3.58%)	2% <ø> (ø)
...ernal/operators/flowable/FlowableFromIterable.java	90.9% <ø> (-3.21%)	5% <ø> (ø)
...ava/io/reactivex/internal/util/HalfSerializer.java	97.5% <ø> (-2.5%)	22% <ø> (-1%)
...ternal/operators/completable/CompletableUsing.java	97.61% <ø> (-2.39%)	4% <ø> (ø)
.../main/java/io/reactivex/subjects/AsyncSubject.java	97.41% <ø> (-1.73%)	55% <ø> (-1%)
...n/java/io/reactivex/processors/AsyncProcessor.java	97.45% <ø> (-1.7%)	55% <ø> (-1%)
...al/operators/observable/ObservableSampleTimed.java	96.66% <ø> (-1.67%)	3% <ø> (ø)
... and 32 more

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