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

A previous commit introduced a new parse_one_advertised_remote()
function that takes a `const char *` argument. This function is called
from filter_promisor_remote() and parses all the fields for one remote.

This means that in filter_promisor_remote() we no longer need to split
the remote information that will be passed to
parse_one_advertised_remote() into an array of relatively heavy and
complex `struct strbuf`.

To use something lighter, let's then replace strbuf_split_str() with
string_list_split() in filter_promisor_remote() to parse the remote
information that is passed to parse_one_advertised_remote().

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