Clean up "performance allocators" and "performance flate2" backends #519

Job	Run time
linux (x86_64-unknown-linux-gnu)	8m 46s
sdist	7m 43s
linux (i686-unknown-linux-gnu)	12m 38s
macos-aarch64	8m 19s
macos-x86_64	9m 40s
windows (x86_64-pc-windows-msvc, x64)	14m 51s
linux-arm (aarch64-unknown-linux-gnu, aarch64, -e JEMALLOC_SYS_WITH_LG_PAGE=16)	9m 42s
linux-powerpc (powerpc64le-unknown-linux-gnu, ppc64le, -e JEMALLOC_SYS_WITH_LG_PAGE=16)	8m 54s
windows (i686-pc-windows-msvc, x86)	15m 57s
linux-arm (armv7-unknown-linux-gnueabihf, armv7)	9m 17s
linux-s390x (s390x-unknown-linux-gnu, s390x)	12m 55s
linux-powerpc (powerpc64-unknown-linux-gnu, ppc64, -e JEMALLOC_SYS_WITH_LG_PAGE=16)	8m 33s
linux-arm (arm-unknown-linux-musleabihf, arm)	8m 12s
musllinux (x86_64-unknown-linux-musl)	9m 2s
musllinux-cross (aarch64-unknown-linux-musl, aarch64, -e JEMALLOC_SYS_WITH_LG_PAGE=16)	10m 7s
musllinux (i686-unknown-linux-musl)	8m 16s
musllinux-cross (armv7-unknown-linux-musleabihf, armv7)	8m 38s
musllinux-cross (powerpc64le-unknown-linux-musl, ppc64le, -e JEMALLOC_SYS_WITH_LG_PAGE=16, nightly)	7m 25s
	2h 58m 55s

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