commit-reach: remove get_reachable_subset() #28333

Job	Run time
config	8s
fuzz smoke test	1m 26s
win+Meson build	6m 54s
sparse	3m 58s
win build	6m 54s
static-analysis	6m 44s
documentation	6m 54s
osx-meson (macos-14)	6m 54s
osx-gcc (macos-14)	6m 54s
osx-clang (macos-14)	6m 55s
rust-analysis	6m 55s
linux-reftable (ubuntu:rolling)	6m 55s
osx-reftable (macos-14)	6m 54s
linux-breaking-changes (ubuntu:rolling)	6m 55s
almalinux-8 (almalinux:8)	6m 55s
linux-leaks (ubuntu:rolling)	6m 55s
linux-reftable-leaks (ubuntu:rolling)	6m 54s
linux32 (i386/ubuntu:focal)	6m 55s
linux-sha256 (ubuntu:rolling)	6m 55s
debian-11 (debian:11)	6m 55s
fedora-breaking-changes-meson (fedora:latest)	6m 55s
win+VS build	0s
linux-meson (ubuntu:rolling)	6m 55s
linux-TEST-vars (ubuntu:20.04)	6m 55s
linux-musl-meson (alpine:latest)	6m 55s
linux-asan-ubsan (ubuntu:rolling)	6m 55s
win+VS test	-1s
win test	0s
win+Meson test	0s
	2h 37m 23s

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