test: mbr: Drop a duplicate test

The test currently runs twice as it is declared twice. Unwind this.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/test/cmd/mbr.c b/test/cmd/mbr.c
index 7279961..b14137e 100644
--- a/test/cmd/mbr.c
+++ b/test/cmd/mbr.c
@@ -479,9 +479,3 @@
 
 	return cmd_ut_category("mbr", "mbr_test_", tests, n_ents, argc, argv);
 }
-
-static int dm_test_cmd_mbr(struct unit_test_state *uts)
-{
-	return mbr_test_run(uts);
-}
-DM_TEST(dm_test_cmd_mbr, UTF_SCAN_FDT | UTF_CONSOLE);