repository: Remove empty cleanup functions in tests

TestRepository and its variants always returned no-op cleanup functions.
If they ever do need to do cleanup, using testing.T.Cleanup is easier
than passing these functions around.

internal/repository/fuzz_test.go

@@ -19,7 +19,7 @@ func FuzzSaveLoadBlob(f *testing.F) {
 		}
 
 		id := restic.Hash(blob)
-		repo, _ := TestRepositoryWithBackend(t, mem.New(), 2)
+		repo := TestRepositoryWithBackend(t, mem.New(), 2)
 
 		var wg errgroup.Group
 		repo.StartPackUploader(context.TODO(), &wg)