remote: delete git_remote_supported_url()

This function does not in fact tell us anything, as almost anything with
a colon in it is a valid rsync-style SSH path; it can not tell us that
we do not support ftp or afp or similar as those are still valid SSH
paths and we do support that.
Merge pull request #2644 from libgit2/cmn/remove-supported-url

remote: delete git_remote_supported_url()