Revert "Chunk gets a ConcurrentHashMap ..."

This reverts commit 0e5f8c4ac697d2f0feec83df292efbb3d0d89d38.
In b25fa4a9c8 and 3411233e65 I moved the getting of the tile entites from
chunks back to the world thread, right before the PacketBuilder job is
queued. Therefore synchronizing on the HashMap in Chunk is not necessary
anymore.