Removes the last of the unprotected variables from Stargate

Makes all classes use getInstance() to get a Stargate instance
Removes the server variable as it's not needed

src/main/java/net/knarcraft/stargate/thread/ChunkUnloadThread.java

@@ -23,7 +23,7 @@ public class ChunkUnloadThread implements Runnable {
             unloadQueue.remove();
             Chunk chunkToUnload = firstElement.getChunkToUnload();
             //Allow the chunk to be unloaded
-            chunkToUnload.removePluginChunkTicket(Stargate.stargate);
+            chunkToUnload.removePluginChunkTicket(Stargate.getInstance());
             firstElement = unloadQueue.peek();
         }
     }