remote: drop checks for zero-url case

remote: drop checks for zero-url case

Now that the previous commit removed the possibility that a "struct
remote" will ever have zero url fields, we can drop a number of
redundant checks and untriggerable code paths.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>