git-sh-setup: remove remnant bits referring to `git-legacy-stash`

In 8a2cd3f512 (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 <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-sh-setup.sh

@@ -101,7 +101,6 @@ $LONG_USAGE")"
 	case "$1" in
 		-h)
 		echo "$LONG_USAGE"
-		case "$0" in *git-legacy-stash) exit 129;; esac
 		exit
 	esac
 fi