Clean up "performance allocators" and "performance flate2" backends #485

Job	Run time
linux-powerpc (powerpc64le-unknown-linux-gnu, ppc64le)	1m 16s
linux-arm (aarch64-unknown-linux-gnu, aarch64, -e JEMALLOC_SYS_WITH_LG_PAGE=16)	2m 0s
linux-s390x (s390x-unknown-linux-gnu, s390x)	6m 56s
linux-powerpc (powerpc64-unknown-linux-gnu, ppc64)	1m 10s
linux-arm (armv7-unknown-linux-gnueabihf, armv7)	2m 0s
linux (x86_64-unknown-linux-gnu)	1m 59s
linux-arm (arm-unknown-linux-musleabihf, arm)	1m 10s
linux (i686-unknown-linux-gnu)	1m 59s
macos-aarch64	1m 8s
macos-x86_64	1m 58s
musllinux (x86_64-unknown-linux-musl)	2m 3s
musllinux-cross (aarch64-unknown-linux-musl, aarch64, -e JEMALLOC_SYS_WITH_LG_PAGE=16)	1m 57s
musllinux (i686-unknown-linux-musl)	1m 14s
musllinux-cross (armv7-unknown-linux-musleabihf, armv7)	2m 23s
sdist	1m 18s
windows (x86_64-pc-windows-msvc, x64)	2m 5s
musllinux-cross (powerpc64le-unknown-linux-musl, ppc64le, nightly)	1m 57s
windows (i686-pc-windows-msvc, x86)	2m 36s
	37m 9s

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