add: remove support for `git-legacy-stash`

add: remove support for `git-legacy-stash`

In 90a6bb98d11 (legacy stash -p: respect the add.interactive.usebuiltin
setting, 2019-12-21), we added support to use the built-in `add -p` from
the scripted `stash -p`.

In 8a2cd3f5123 (stash: remove the stash.useBuiltin setting, 2020-03-03),
we retired the scripted `stash` (including the scripted `stash -p`).

Therefore this support is no longer necessary.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>