send-email: move "hooks_path" invocation to git-send-email.perl

send-email: move "hooks_path" invocation to git-send-email.perl

Move the newly added "hooks_path" API in Git.pm to its only user in
git-send-email.perl. This was added in c8243933c74 (git-send-email:
Respect core.hooksPath setting, 2021-03-23), meaning that it hasn't
yet made it into a non-rc release of git.

The consensus with Git.pm is that we need to be considerate of
out-of-tree users who treat it as a public documented interface. We
should therefore be less willing to add new functionality to it, least
we be stuck supporting it after our own uses for it disappear.

In this case the git-send-email.perl hook invocation will probably be
replaced by a future "git hook run" command, and in the commit
preceding this one the "hooks_path" become nothing but a trivial
wrapper for "rev-parse --git-path hooks" anyway (with no
Cwd::abs_path() call), so let's just inline this command in
git-send-email.perl itself.

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