submodule.sh: remove unused variable

This could have been part of 4830868 (git submodule update: have a
dedicated helper for cloning, 2016-02-29).

Signed-off-by: Stefan Beller <sbeller@google.com>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: stefanbeller

git-submodule.sh

@@ -596,7 +596,6 @@ cmd_update()
 		die_if_unmatched "$mode" "$sha1"
 
 		name=$(git submodule--helper name "$sm_path") || exit
-		url=$(git config submodule."$name".url)
 		if ! test -z "$update"
 		then
 			update_module=$update