Revert 2124b5ec, suspect that event firing may be broken

proxy/src/main/java/com/velocitypowered/proxy/event/EventTypeTracker.java

@@ -44,7 +44,6 @@ class EventTypeTracker {
       }
 
       friends.put(type, eventType);
-      friends.put(eventType, type);
     }
     return types;
   }