Clean up "performance allocators" and "performance flate2" backends #512

Job	Run time
linux-s390x (s390x-unknown-linux-gnu, s390x)	3m 41s
linux (x86_64-unknown-linux-gnu)	3m 42s
linux (i686-unknown-linux-gnu)	3m 42s
linux-powerpc (powerpc64le-unknown-linux-gnu, ppc64le, -e JEMALLOC_SYS_WITH_LG_PAGE=16)	3m 43s
linux-arm (aarch64-unknown-linux-gnu, aarch64, -e JEMALLOC_SYS_WITH_LG_PAGE=16)	3m 43s
macos-aarch64	3m 43s
linux-powerpc (powerpc64-unknown-linux-gnu, ppc64, -e JEMALLOC_SYS_WITH_LG_PAGE=16)	3m 40s
linux-arm (armv7-unknown-linux-gnueabihf, armv7)	3m 40s
macos-x86_64	3m 41s
linux-arm (arm-unknown-linux-musleabihf, arm)	3m 40s
musllinux (x86_64-unknown-linux-musl)	3m 41s
musllinux-cross (aarch64-unknown-linux-musl, aarch64, -e JEMALLOC_SYS_WITH_LG_PAGE=16)	3m 41s
sdist	3m 53s
musllinux (i686-unknown-linux-musl)	3m 41s
musllinux-cross (armv7-unknown-linux-musleabihf, armv7)	3m 42s
windows (x86_64-pc-windows-msvc, x64)	3m 40s
musllinux-cross (powerpc64le-unknown-linux-musl, ppc64le, -e JEMALLOC_SYS_WITH_LG_PAGE=16, nightly)	3m 40s
windows (i686-pc-windows-msvc, x86)	3m 41s
	1h 6m 34s

Provide feedback