Remove $create parameter from ChunkManager::getChunk()

this restores SimpleChunkManager's behaviour to PM3, removing the need for GeneratorChunkManager (although I'm dubious whether SubChunkExplorer makes any sense in there any more now that we have morton in the mix).

src/world/ChunkManager.php

@@ -40,7 +40,7 @@ interface ChunkManager{
 	 */
 	public function setBlockAt(int $x, int $y, int $z, Block $block) : void;
 
-	public function getChunk(int $chunkX, int $chunkZ, bool $create = false) : ?Chunk;
+	public function getChunk(int $chunkX, int $chunkZ) : ?Chunk;
 
 	public function setChunk(int $chunkX, int $chunkZ, ?Chunk $chunk) : void;