gsm0408_test: sanitize: cleanup msgb

Remove initial msgb talloc context creation: if we create a root ctx for msgb
that all msgb are allocated in, we would in a final cleanup discard all msgbs,
i.e. we would not verify that all msgb are cleaned up properly.

If we create the msgb context and *don't* clean it up in the end, the sanitizer
build fails because the context root is not cleaned up.

Easiest is to actually allocate all msgb at NULL ctx, because then any msgb
that aren't cleaned up properly would still linger, while we don't leave a root
ctx that we need to clean up either.

Helps fix sanitizer build on debian 9.

Change-Id: I1f2d1d05c75bbf4d92787f9735083f18cdc90f6f
diff --git a/tests/gsm0408/gsm0408_test.c b/tests/gsm0408/gsm0408_test.c
index 3f3a5c7..935ec21 100644
--- a/tests/gsm0408/gsm0408_test.c
+++ b/tests/gsm0408/gsm0408_test.c
@@ -197,7 +197,6 @@
 
 int main(int argc, char **argv)
 {
-	msgb_talloc_ctx_init(NULL, 0);
 	test_bearer_cap();
 	test_mid_from_tmsi();
 	test_ra_cap();