Remove dead code of hand-rolled iterators

[bartdesmet]

Remove dead code predicated by !USE_ASYNC_ITERATOR given we've switched over to use iterators across the board.

[codecov]

Codecov Report

Merging #905 into master will increase coverage by 3.12%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master     #905      +/-   ##
==========================================
+ Coverage   62.24%   65.37%   +3.12%     
==========================================
  Files         165      165              
  Lines       32331    25436    -6895     
  Branches    24399    18438    -5961     
==========================================
- Hits        20124    16628    -3496     
+ Misses       2565     2069     -496     
+ Partials     9642     6739    -2903

Flag	Coverage Δ
#ixnet	65.37% <50%> (+3.12%)	⬆️

Impacted Files	Coverage Δ
...tem.Interactive.Async/System/Linq/Operators/Amb.cs	34.63% <ø> (-8.42%)	⬇️
...ce/System.Linq.Async/System/Linq/Operators/Cast.cs	76.92% <ø> (+2.53%)	⬆️
....Interactive.Async/System/Linq/Operators/Repeat.cs	57.62% <ø> (+10.35%)	⬆️
...stem.Interactive.Async/System/Linq/Operators/Do.cs	31.27% <ø> (-11.1%)	⬇️
...ystem.Linq.Async/System/Linq/Operators/SkipLast.cs	77.55% <ø> (+1.94%)	⬆️
...em.Interactive.Async/System/Linq/Operators/Scan.cs	38.55% <ø> (-7.02%)	⬇️
...stem.Linq.Async/System/Linq/Operators/SkipWhile.cs	67.69% <ø> (+10.12%)	⬆️
.../System.Linq.Async/System/Linq/Operators/OfType.cs	79.31% <ø> (+5.15%)	⬆️
...m.Interactive.Async/System/Linq/Operators/Using.cs	55.07% <ø> (+14.65%)	⬆️
...tem.Linq.Async/System/Linq/Operators/SelectMany.cs	58.89% <ø> (+0.76%)	⬆️
... and 111 more

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