git-sh-setup: remove unused sane_egrep() function

git-sh-setup: remove unused sane_egrep() function

The is_zero_oid() function in git-submodule.sh has not been used since
e83e3333b57 (submodule: port submodule subcommand 'summary' from shell
to C, 2020-08-13), so we can remove it, and the sane_egrep() function,
dead is_zero_oid() was the only function which still referenced it.

Unlike some other functions in git-sh-setup.sh, this function has not
been documented in git-sh-setup(1), so per [1] it should be OK to
remove it. I'm still unclear about the future of some of the other
functions[2], but any questions in that area should not apply here.

1. https://lore.kernel.org/git/xmqqr1dtgnn8.fsf@gitster.g/
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>