remote: get rid of git_remote_valid_url()

It does the same as git_remote_supported_url() but has a name which
implies we'd check the URL for correctness while we're simply looking at
the scheme and looking it up in our lists.

While here, fix up the tests so we check all the combination of what's
supported.
remote: test for supported URLs in a single place

Instead of using ifdefs to run the tests, use them to set when we expect
to support a particular scheme and always have the tests in the code.