tests: posix: pthread: remove unused pthread return value

Rather than pass an variable address to a `void *` in
`pthread_join()` and do nothing with it, just pass `NULL`.

Signed-off-by: Christopher Friedt <[email protected]>
(cherry picked from commit 12ed08a)

Author: cfriedt

tests/posix/common/src/pthread.c

@@ -563,7 +563,6 @@ void test_posix_pthread_create_negative(void)
 
 void test_pthread_descriptor_leak(void)
 {
-	void *unused;
 	pthread_t pthread1;
 	pthread_attr_t attr;