lib: Get rid of unused tst_require_root() callback

Signed-off-by: Cyril Hrubis <chrubis@suse.cz>
diff --git a/testcases/kernel/mem/oom/oom02.c b/testcases/kernel/mem/oom/oom02.c
index fce0110..48bd0a3 100644
--- a/testcases/kernel/mem/oom/oom02.c
+++ b/testcases/kernel/mem/oom/oom02.c
@@ -75,7 +75,7 @@
 
 void setup(void)
 {
-	tst_require_root(NULL);
+	tst_require_root();
 	tst_sig(FORK, DEF_HANDLER, cleanup);
 	TEST_PAUSE;