Don't put cursor at the end of wxGridCellEnumEditor control.

Don't put cursor at the end of wxGridCellEnumEditor control.

Putting the cursor to the end of the control when the editing starts doesn't
make much sense as this should be the default behaviour anyhow and, worse,
this results in an assert under wxMSW where a read-only wxComboBox doesn't
have any cursor to move.

Closes #12446.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66143 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775