3.x: Remove Changes.md

[akarnokd]

An unnecessary duplication of the releases tab. Changing the file triggers an unecessary build that has to be canceled on Travis CI so that the actual release runs without interference.

Resolves #5646

[codecov]

Codecov Report

Merging #6533 into 3.x will decrease coverage by 0.06%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                3.x    #6533      +/-   ##
============================================
- Coverage     98.26%   98.19%   -0.07%     
- Complexity     6185     6189       +4     
============================================
  Files           680      680              
  Lines         44883    44883              
  Branches       6193     6193              
============================================
- Hits          44106    44075      -31     
- Misses          247      261      +14     
- Partials        530      547      +17

Impacted Files	Coverage Δ	Complexity Δ
...l/operators/observable/ObservableFlatMapMaybe.java	86.92% <0%> (-7.19%)	2% <0%> (ø)
.../operators/observable/ObservableFlatMapSingle.java	88.8% <0%> (-2.99%)	2% <0%> (ø)
...rnal/operators/flowable/FlowableFlatMapSingle.java	93.47% <0%> (-2.18%)	2% <0%> (ø)
...rnal/operators/flowable/FlowableSkipLastTimed.java	95.91% <0%> (-2.05%)	2% <0%> (ø)
...nternal/operators/observable/ObservableWindow.java	98% <0%> (-2%)	3% <0%> (ø)
...nternal/operators/parallel/ParallelReduceFull.java	91.08% <0%> (-1.99%)	2% <0%> (ø)
...tivex/internal/operators/single/SingleTimeout.java	98.33% <0%> (-1.67%)	2% <0%> (ø)
...rnal/operators/observable/ObservableSwitchMap.java	94.68% <0%> (-1.6%)	3% <0%> (ø)
...a/io/reactivex/internal/util/QueueDrainHelper.java	98.61% <0%> (-1.39%)	57% <0%> (-1%)
...ternal/operators/observable/ObservableFlatMap.java	88.67% <0%> (-1.3%)	3% <0%> (ø)
... and 23 more

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