Remove assert checking for non zero page size in wxDataViewCtrl.

This assert seems to be quite useless and can be triggered if a key is
(perhaps accidentally) sent to a small (e.g. not yet fully laid out)
wxDataViewCtrl window.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@78146 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775