git-submodule: remove unused is_zero_oid() function

The is_zero_oid() function in git-submodule.sh has not been used since
e83e3333b5 (submodule: port submodule subcommand 'summary' from shell
to C, 2020-08-13), so we can remove it.

This was the last user of the sane_egrep() function in
git-sh-setup.sh. I'm not removing it in case some out-of-tree user
relied on it. Per the discussion that can be found upthread of [1].

1. https://lore.kernel.org/git/87tuiwjfvi.fsf@evledraar.gmail.com/

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-submodule.sh

@@ -63,11 +63,6 @@ isnumber()
 	n=$(($1 + 0)) 2>/dev/null && test "$n" = "$1"
 }
 
-# Given a full hex object ID, is this the zero OID?
-is_zero_oid () {
-	echo "$1" | sane_egrep '^0+$' >/dev/null 2>&1
-}
-
 # Sanitize the local git environment for use within a submodule. We
 # can't simply use clear_local_git_env since we want to preserve some
 # of the settings from GIT_CONFIG_PARAMETERS.