Some minor cleanup

[bartdesmet]

Remove some unused using and use shadowing of local function parameters in one more case.

[codecov]

Codecov Report

Merging #906 into master will increase coverage by 0.14%.
The diff coverage is 37.5%.

@@            Coverage Diff             @@
##           master     #906      +/-   ##
==========================================
+ Coverage   65.37%   65.51%   +0.14%     
==========================================
  Files         165      165              
  Lines       25436    25429       -7     
  Branches    18438    18351      -87     
==========================================
+ Hits        16628    16661      +33     
- Misses       2069     2132      +63     
+ Partials     6739     6636     -103

Flag	Coverage Δ
#ixnet	65.51% <37.5%> (+0.14%)	⬆️

Impacted Files	Coverage Δ
...ce/System.Linq.Async/System/Linq/Operators/Join.cs	72.89% <ø> (+1.03%)	⬆️
...stem.Linq.Async/System/Linq/Operators/Intersect.cs	78.78% <ø> (+2.31%)	⬆️
...stem.Linq.Async/System/Linq/Operators/SkipWhile.cs	68% <ø> (+0.3%)	⬆️
...stem.Linq.Async/System/Linq/Operators/TakeWhile.cs	73.33% <ø> (+1.64%)	⬆️
....Linq.Async/System/Linq/Operators/SequenceEqual.cs	76.27% <100%> (+2.07%)	⬆️
...tem.Linq.Async/System/Linq/Operators/SelectMany.cs	59.03% <28.57%> (+0.14%)	⬆️
.../System.Linq.Async/System/Linq/Operators/Repeat.cs	72.61% <0%> (-1.2%)	⬇️
...rce/System.Linq.Async/System/Linq/Operators/Zip.cs	74.31% <0%> (ø)	⬆️
...System.Linq.Async/System/Linq/Operators/Reverse.cs	73.58% <0%> (ø)	⬆️
...rce/System.Linq.Async/System/Linq/Operators/Any.cs	74.15% <0%> (ø)	⬆️
... and 38 more

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