2.x Wiki: Remove mention of i.r.f.Functions

[akarnokd]

The io.reactivex.functions.Functions utility method has been made internal a long ago and should not be mentioned.

Resolves: #6239

[codecov]

Codecov Report

Merging #6241 into 2.x will increase coverage by 0.1%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##                2.x    #6241     +/-   ##
===========================================
+ Coverage     98.22%   98.32%   +0.1%     
- Complexity     6199     6206      +7     
===========================================
  Files           667      667             
  Lines         44887    44887             
  Branches       6216     6216             
===========================================
+ Hits          44089    44134     +45     
+ Misses          255      237     -18     
+ Partials        543      516     -27

Impacted Files	Coverage Δ	Complexity Δ
.../io/reactivex/disposables/CompositeDisposable.java	98.14% <0%> (-1.86%)	39% <0%> (-1%)
...rnal/operators/observable/ObservableSwitchMap.java	94.68% <0%> (-1.6%)	3% <0%> (ø)
...rnal/operators/flowable/FlowableFlatMapSingle.java	95.65% <0%> (-1.09%)	2% <0%> (ø)
...main/java/io/reactivex/subjects/ReplaySubject.java	97.69% <0%> (-0.42%)	50% <0%> (ø)
...ternal/operators/observable/ObservableFlatMap.java	89.61% <0%> (-0.33%)	3% <0%> (ø)
...ternal/operators/flowable/FlowableWindowTimed.java	91.4% <0%> (+0.22%)	4% <0%> (ø)	⬇️
...internal/operators/flowable/FlowableConcatMap.java	98.12% <0%> (+0.37%)	6% <0%> (ø)	⬇️
...va/io/reactivex/processors/MulticastProcessor.java	98.67% <0%> (+0.44%)	84% <0%> (+1%)	⬆️
...internal/operators/flowable/FlowableSwitchMap.java	97.16% <0%> (+0.47%)	3% <0%> (ø)	⬇️
...internal/operators/flowable/FlowableObserveOn.java	97.21% <0%> (+0.55%)	3% <0%> (ø)	⬇️
... and 20 more

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