In 8a2cd3f5123 (stash: remove the stash.useBuiltin setting, 2020-03-03),
we removed `git-legacy-stash.sh`. But `git-sh-setup.sh` somehow still
thinks about it. Let's just not.
Signed-off-by: Johannes Schindelin <redacted> Signed-off-by: Junio C Hamano <redacted>