Remove extraneous build files #4511

Job	Run time
build-macos	5m 34s
build-freebsd	16m 30s
build-sanitizers (-DMOLD_USE_TSAN=On)	24m 35s
build-multi-archs	20m 42s
build-windows	22m 30s
build-sanitizers (-DMOLD_USE_ASAN=On)	14m 16s
build-distros (archlinux)	17m 28s
build-msys	11m 54s
build-distros (debian:11)	11m 43s
build-distros (gentoo/stage3)	21m 17s
build-distros (fedora)	12m 18s
build-distros (opensuse/tumbleweed)	13m 16s
build-sanitizers	10m 54s
build-distros (alpine)	17m 5s
build-distros (ubuntu:22.04)	11m 19s
	3h 51m 21s

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