hook: move unsorted_string_list_remove() to string-list.[ch]

hook: move unsorted_string_list_remove() to string-list.[ch]

Move the convenience wrapper from hook to string-list since
it's a more suitable place. Add a doc comment to the header.

Also add a free_util arg to make the function more generic
and make the API similar to other functions in string-list.h.
Update the existing call-sites.

Suggested-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Adrian Ratiu <adrian.ratiu@collabora.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>