Avoid creating wxGenericNotificationMessageImpl twice

In the case where native wxNotificationMessage is not available and
wxGenericNotificationMessage is used as wxNotificationMessage, m_impl was
being initialized twice.  This happens already in the constructor for
wxGenericNotificationMessage.  This was causing a leak of a
wxGenericNotificationMessageImpl and even more problematic, a leak of a
wxFrame, which caused some oddities.  So, just remove the initialization from
wxNotificationMessage.

Closes https://github.com/wxWidgets/wxWidgets/pull/2631

src/common/notifmsgcmn.cpp

@@ -91,7 +91,10 @@ bool wxNotificationMessageBase::AddAction(wxWindowID actionid, const wxString &l
 
 void wxNotificationMessage::Init()
 {
-    m_impl = new wxGenericNotificationMessageImpl(this);
+    // This is only used when we derive from wxGenericNotificationMessage,
+    // which already initializes its m_impl correctly, so there is nothing to
+    // do here (but we still need to have this method for consistency with the
+    // native implementations).
 }
 
 #endif