Correctly implement SelectedSelectionKeySet.Iterator remove()

Motivation:

We need to implement remove() by ourselves to make it work on Java7 as otherwise it will throw an AbstractMethodError. This is a followup of c1a335446d.

Modifications:

Just implemented remove()

Result:

Works on Java7 as well.

transport/src/main/java/io/netty/channel/nio/SelectedSelectionKeySet.java

@@ -76,6 +76,11 @@ final class SelectedSelectionKeySet extends AbstractSet<SelectionKey> {
                 }
                 return keys[idx++];
             }
+
+            @Override
+            public void remove() {
+                throw new UnsupportedOperationException();
+            }
         };
     }