etc: Do not overestimate number of CPU threads

The *2 factor between cores and threads of most modern CPUs
is already taken into account in get_processor_count().

modules/etc/image_etc.cpp

@@ -138,7 +138,7 @@ static void _compress_etc(Image *p_img, float p_lossy_quality, bool force_etc1_f
 	PoolVector<uint8_t>::Write w = dst_data.write();
 
 	// prepare parameters to be passed to etc2comp
-	int num_cpus = OS::get_singleton()->get_processor_count() * 2; //generally some cpus have 2 threads
+	int num_cpus = OS::get_singleton()->get_processor_count();
 	int encoding_time = 0;
 	float effort = 0.0; //default, reasonable time