lib: Get rid of unused tst_require_root() callback

Signed-off-by: Cyril Hrubis <chrubis@suse.cz>
diff --git a/testcases/kernel/mem/oom/oom01.c b/testcases/kernel/mem/oom/oom01.c
index 44fa908..712d7b5 100644
--- a/testcases/kernel/mem/oom/oom01.c
+++ b/testcases/kernel/mem/oom/oom01.c
@@ -74,7 +74,7 @@
 
 void setup(void)
 {
-	tst_require_root(NULL);
+	tst_require_root();
 	tst_sig(FORK, DEF_HANDLER, cleanup);
 	TEST_PAUSE;
 
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;
 
diff --git a/testcases/kernel/mem/oom/oom03.c b/testcases/kernel/mem/oom/oom03.c
index c5d20b3..3afc3de 100644
--- a/testcases/kernel/mem/oom/oom03.c
+++ b/testcases/kernel/mem/oom/oom03.c
@@ -90,7 +90,7 @@
 
 void setup(void)
 {
-	tst_require_root(NULL);
+	tst_require_root();
 	tst_sig(FORK, DEF_HANDLER, cleanup);
 	TEST_PAUSE;
 
diff --git a/testcases/kernel/mem/oom/oom04.c b/testcases/kernel/mem/oom/oom04.c
index 3defe91..ffb5503 100644
--- a/testcases/kernel/mem/oom/oom04.c
+++ b/testcases/kernel/mem/oom/oom04.c
@@ -83,7 +83,7 @@
 {
 	int memnode, ret;
 
-	tst_require_root(NULL);
+	tst_require_root();
 	tst_sig(FORK, DEF_HANDLER, cleanup);
 	TEST_PAUSE;
 
diff --git a/testcases/kernel/mem/oom/oom05.c b/testcases/kernel/mem/oom/oom05.c
index 38f7e14..9cd49a7 100644
--- a/testcases/kernel/mem/oom/oom05.c
+++ b/testcases/kernel/mem/oom/oom05.c
@@ -106,7 +106,7 @@
 {
 	int ret, memnode;
 
-	tst_require_root(NULL);
+	tst_require_root();
 	tst_sig(FORK, DEF_HANDLER, cleanup);
 	TEST_PAUSE;