tests: Do not explicitly set the thread pointer to NULL

FRR should only ever use the appropriate THREAD_ON/THREAD_OFF
semantics.  This is espacially true for the functions we
end up calling the thread for.

Signed-off-by: Donatas Abraitis <donatas.abraitis@gmail.com>

tests/lib/test_timer_correctness.c

@@ -153,7 +153,6 @@ int main(int argc, char **argv)
 
 		XFREE(MTYPE_TMP, timers[index]->arg);
 		thread_cancel(&timers[index]);
-		timers[index] = NULL;
 		timers_pending--;
 	}
 

tests/lib/test_timer_performance.c

@@ -77,7 +77,6 @@ int main(int argc, char **argv)
 
 		index = prng_rand(prng) % SCHEDULE_TIMERS;
 		thread_cancel(&timers[index]);
-		timers[index] = NULL;
 	}
 
 	monotime(&tv_stop);