Remove non-null check for world of Player entity

uranos-server/src/main/kotlin/space/uranos/entity/UranosEntity.kt

@@ -34,9 +34,6 @@ sealed class UranosEntity(server: UranosServer) : Entity {
     final override var world: World? = null; private set
 
     override suspend fun setWorld(world: World?) {
-        if (world == null && this is PlayerEntity)
-            throw IllegalArgumentException("You cannot set the world of a PlayerEntity to null")
-
         if (world == this.world) return
 
         worldMutex.withLock {