Revert "LibGUI: Ignore the Enter event by default"

This reverts commit a5a32fbccef8f8f663001363a278f80ef6320efe.

This change was wrong: it's fine for a child widget to receive an enter
event despite its parent never getting one. Only the widget directly
under the mouse cursor is considered hovered for enter/leave purposes.

Userland/Libraries/LibGUI/Widget.cpp

@@ -528,9 +528,8 @@ void Widget::focusout_event(FocusEvent&)
 {
 }
 
-void Widget::enter_event(Core::Event& event)
+void Widget::enter_event(Core::Event&)
 {
-    event.ignore();
 }
 
 void Widget::leave_event(Core::Event&)