Do not mark selected wxPG property as 'being deleted'.

Only unselected wxPG property can be marked as 'being deleted'.
If this is done for selected wxGP property then in some circumstances it cannot be unselected and hence cannot be safely deleted.

See #16617.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@78091 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775