Entity: make getNetworkTypeId non-static (#6037)

This was static to permit ItemFactory to register spawn eggs for all known entity types in early PM4. However, nowadays we provide a callback to the spawn egg instead, and spawn eggs must be manually implemented, so this is no longer needed.

In addition, having this static forces everyone to make a new entity class for every unique type of entity, which isn't ideal.

src/world/sound/EntityShortFallSound.php

@@ -39,7 +39,7 @@ class EntityShortFallSound implements Sound{
 			LevelSoundEvent::FALL_SMALL,
 			$pos,
 			-1,
-			$this->entity::getNetworkTypeId(),
+			$this->entity->getNetworkTypeId(),
 			false, //TODO: does isBaby have any relevance here?
 			false
 		)];