git-diff-tree: don't use diffcore_pathspec()

diff-tree does the culling of uninteresting paths internally, and
fundamentally has to do so for performance reasons. So there's no
point in calling the separate pathname culling logic here,
especially as it seems slightly broken.

diff-tree.c

@@ -268,8 +268,6 @@ static int call_diff_flush(void)
 		diff_flush(DIFF_FORMAT_NO_OUTPUT, 0);
 		return 0;
 	}
-	if (nr_paths)
-		diffcore_pathspec(paths);
 	if (header) {
 		if (diff_output_format == DIFF_FORMAT_MACHINE) {
 			const char *ep, *cp;