transport: return ENOTFOUND for HTTPS and SSH when they're not supported

The previous commit makes it harder to figure out if the library was
built with support for a particular transport. Roll back some of the
changes and remove ssh:// and https:// from the list if we're being
built without support for them.
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 schema and looking it up in our lists.