Clean up "performance allocators" and "performance flate2" backends #478

Job	Run time
linux-s390x (s390x-unknown-linux-gnu, s390x)	13m 35s
linux-arm (aarch64-unknown-linux-gnu, aarch64, -e JEMALLOC_SYS_WITH_LG_PAGE=16)	8m 51s
linux (x86_64-unknown-linux-gnu)	8m 15s
linux-arm (armv7-unknown-linux-gnueabihf, armv7)	8m 39s
linux-powerpc (powerpc64le-unknown-linux-gnu, ppc64le)	1m 48s
linux (i686-unknown-linux-gnu)	12m 36s
macos-aarch64	7m 8s
linux-arm (arm-unknown-linux-musleabihf, arm)	8m 14s
linux-powerpc (powerpc64-unknown-linux-gnu, ppc64)	1m 56s
macos-x86_64	9m 42s
sdist	36s
windows (x86_64-pc-windows-msvc, x64)	14m 55s
windows (i686-pc-windows-msvc, x86)	14m 18s
musllinux-cross (aarch64-unknown-linux-musl, aarch64, -e JEMALLOC_SYS_WITH_LG_PAGE=16)	9m 47s
musllinux-cross (armv7-unknown-linux-musleabihf, armv7)	8m 26s
musllinux (x86_64-unknown-linux-musl)	8m 34s
musllinux-cross (powerpc64le-unknown-linux-musl, ppc64le, nightly)	6m 59s
musllinux (i686-unknown-linux-musl)	7m 49s
	2h 32m 8s

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