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/utility/PermissionHelper.java

@@ -81,7 +81,7 @@ public final class PermissionHelper {
      */
     public static boolean portalAccessDenied(Player player, Portal portal, boolean deny) {
         StargateAccessEvent event = new StargateAccessEvent(player, portal, deny);
-        Stargate.server.getPluginManager().callEvent(event);
+        Stargate.getInstance().getServer().getPluginManager().callEvent(event);
         return event.getDeny();
     }