git-submodule: remove redundant call to git-describe

The code to find a more descriptive name given a commit in a
submodule were improved in bffe71f4, but it forgot to remove the
older logic the patch replaced.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

git-submodule.sh

@@ -236,7 +236,6 @@ modules_list()
 			say "-$sha1 $path"
 			continue;
 		fi
-		revname=$(unset GIT_DIR && cd "$path" && git describe --tags $sha1)
 		set_name_rev "$path" "$sha1"
 		if git diff-files --quiet -- "$path"
 		then