git-submodule - Use "get_default_remote" from git-parse-remote

git-submodule - Use "get_default_remote" from git-parse-remote

Resolve_relative_url was using its own code for this function, but
this is duplication with the best result that this continues to work.
Replace with the common function provided by git-parse-remote.

Signed-off-by: Mark Levedahl <mlevedahl@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>