Rm parallel::retain_unordered_mut #1410

Job	Run time
Test CUDA support	50s
Test build-std (aarch64-tvos)	2m 26s
Clippy	9s
Test build-std (aarch64-tvos-sim)	2m 28s
MSRV (ubuntu-latest)	1m 33s
Rustfmt	13s
Test build-std (x86_64-tvos)	2m 21s
MSRV (windows-latest)	1m 58s
Test (stable)	52s
Test (beta)	59s
Test (nightly)	1m 1s
Test (linux32)	1m 9s
Test (macos)	1m 5s
Test (aarch64-macos)	57s
Test (aarch64-ios)	1m 5s
Test (win32)	58s
Test (win64)	55s
Test (mingw32)	54s
Test (mingw64)	53s
Test (windows-2019)	53s
Test (windows-aarch64, windows-latest, stable, aarch64-pc-windows-msvc, --no-run)	54s
Test (windows-clang, windows-2019, stable, x86_64-pc-windows-msvc, clang, clang++)	56s
Test (windows-clang-cl, windows-2019, stable, x86_64-pc-windows-msvc, clang-cl, clang-cl)	54s
	26m 23s

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