Remove selection showing code from the grid sample.

Remove selection showing code from the grid sample.

This code is broken as it doesn't always show the selection correctly and
doesn't handle rows-or-columns selection mode at all. Until we can fix it
properly it's better to not have it at all so that at least people avoid
copying the wrong code into their own programs.

Closes #12195.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65075 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775