t5318-commit-graph: remove unused variable

This is a remnant from early versions of the commit-graph patch series
[1], when 'git commit-graph --write' printed the hash of the created
commit-graph file, and tests did look at the command's output, because
the commit-graph file's name included that hash as well.

[1] https://public-inbox.org/git/[email protected]/

Signed-off-by: SZEDER Gábor <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: szeder

t/t5318-commit-graph.sh

@@ -75,7 +75,7 @@ graph_read_expect() {
 
 test_expect_success 'write graph' '
 	cd "$TRASH_DIRECTORY/full" &&
-	graph1=$(git commit-graph write) &&
+	git commit-graph write &&
 	test_path_is_file $objdir/info/commit-graph &&
 	graph_read_expect "3"
 '