Remove unnecessary generic parameters

[bensandee]

Remove unnecessary generic parameters for CompletableConsumers and remove erroneous references to Maybe in javadocs.

[codecov]

Codecov Report

Merging #21 into master will decrease coverage by 0.02%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master      #21      +/-   ##
============================================
- Coverage     82.94%   82.91%   -0.03%     
+ Complexity     2168     2164       -4     
============================================
  Files           356      356              
  Lines         15485    15485              
  Branches       1918     1918              
============================================
- Hits          12844    12840       -4     
  Misses         1995     1995              
- Partials        646      650       +4

Impacted Files	Coverage Δ	Complexity Δ
...arnokd/rxjava2/consumers/CompletableConsumers.java	100% <100%> (ø)	3 <0> (ø)	⬇️
...in/java/hu/akarnokd/rxjava2/util/SpscOneQueue.java	50% <0%> (-7.15%)	4% <0%> (-1%)
...in/java/hu/akarnokd/rxjava2/joins/ActivePlan7.java	78% <0%> (-6%)	5% <0%> (-3%)
...in/java/hu/akarnokd/rxjava2/joins/ActivePlan3.java	85.71% <0%> (-4.77%)	4% <0%> (-1%)
...in/java/hu/akarnokd/rxjava2/joins/ActivePlan4.java	78.57% <0%> (-3.58%)	4% <0%> (-1%)
...in/java/hu/akarnokd/rxjava2/joins/ActivePlan5.java	84.21% <0%> (-2.64%)	6% <0%> (-1%)
.../java/hu/akarnokd/rxjava2/joins/JoinObserver1.java	70.96% <0%> (-1.62%)	11% <0%> (ø)
...in/java/hu/akarnokd/rxjava2/joins/ActivePlan9.java	80.64% <0%> (-1.62%)	8% <0%> (-1%)
...rnokd/rxjava2/basetypes/BlockingGetSubscriber.java	39.24% <0%> (-1.27%)	10% <0%> (-1%)
...akarnokd/rxjava2/schedulers/ParallelScheduler.java	94.38% <0%> (+0.56%)	32% <0%> (ø)	⬇️
... and 3 more

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