Use wxLocale::GetInfo() to get date format in wxDatePickerCtrlGeneric.

Use wxLocale::GetInfo() to get date format in wxDatePickerCtrlGeneric.

Instead of trying (and failing) to manually decode the format used by "%x",
simply use wxLocale::GetInfo(wxLOCALE_SHORT_DATE_FMT) and just change "%y" to
"%Y" if necessary.

The new code is more than 10 times shorter and also gives correct results
unlike the old version.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65511 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775