Remove wxLOCALE_CONV_ENCODING flag.

This doesn't affect Unicode builds, only the legacy ANSI one.

PO files are now always converted to the appropriate charset according
to current locale. This was the default behavior for long time; not
doing the conversion was always a fragile hack that didn't work well.
Removing this flag simplifies wxLocale code.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63993 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775