Small clean-ups in event manager.

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

@@ -33,8 +33,9 @@ class EventTypeTracker {
   }
 
   public Collection<Class<?>> getFriendsOf(final Class<?> eventType) {
-    if (friends.containsKey(eventType)) {
-      return friends.get(eventType);
+    ImmutableSet<Class<?>> existingFriends = friends.get(eventType);
+    if (existingFriends != null) {
+      return existingFriends;
     }
 
     final Collection<Class<?>> types = getEventTypes(eventType);

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

@@ -156,17 +156,6 @@ public class VelocityEventManager implements EventManager {
     }
   }
 
-  enum AsyncType {
-    /**
-     * The complete event will be handled on an async thread.
-     */
-    ALWAYS,
-    /**
-     * The event will never run async, everything is handled on the netty thread.
-     */
-    NEVER
-  }
-
   static final class HandlersCache {
 
     final HandlerRegistration[] handlers;