Remove the lastChunkAccessed if it is unloaded.

This fixes an issue where a chunk would be unloaded but remain in lastChunkAccessed meaning calls on getChunkAt could return a chunk that is no longer loaded, this caused an issue where the chunk could be reloaded whilst in use reverting any block changes. This caused findEndPortal to return null even after createEndPortal which would crash the server trying to teleport to a null location.