Removes unneed sync

sources/src/main/java/io/akarin/server/mixin/core/NonblockingServerConnection.java

@@ -159,9 +159,7 @@ public class NonblockingServerConnection {
                 if (manager.preparing) continue;
                 
                 needRemoval = true;
-                synchronized (pending) {
                 pending.add(manager);
-                }
                 
                 manager.handleDisconnection();
             }