promisor-remote: use string_list_split() in mark_remotes_as_accepted()

promisor-remote: use string_list_split() in mark_remotes_as_accepted()

Previous commits replaced some strbuf_split*() calls with calls to
string_list_split*() in "promisor-remote.c".

For consistency, let's also replace the strbuf_split_str() call in
mark_remotes_as_accepted() with a call to string_list_split(), as we
don't need the splitted strings to be managed by a `struct strbuf`.
Using the lighter-weight `string_list` API is enough for our needs.

While at it let's remove a useless call to `strbuf_strip_suffix()`.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>