Don't skip event after capturing mouse in wxAUI in all ports

This was previously done only under Mac, but it doesn't seem to make
much sense to skip the mouse event if we handled it by capturing the
mouse and it seems to create problems in wxQt too, so just generalize
the Mac behaviour to all platforms.

See https://github.com/wxWidgets/wxWidgets/pull/1360

src/aui/framemanager.cpp

@@ -4160,21 +4160,15 @@ void wxAuiManager::OnLeftDown(wxMouseEvent& event)
                                       event.m_y - part->rect.y);
             m_frame->CaptureMouse();
         }
-#ifdef __WXMAC__
         else
         {
             event.Skip();
         }
-#endif
     }
-#ifdef __WXMAC__
     else
     {
         event.Skip();
     }
-#else
-    event.Skip();
-#endif
 }
 
 /// Ends a resize action, or for live update, resizes the sash