Work around wxDateTime::ParseFormat() problem in generic wxDateTimePickerCtrl.

Work around wxDateTime::ParseFormat() problem in generic wxDateTimePickerCtrl.

Due to a bug in wxDateTime::ParseFormat(), calling it with wxString::c_str()
currently just crashes, so don't do this in generic wxDateTimePickerCtrl code.
The real fix is, of course, to correct the bug in wxDateTime itself but for
now this at least prevents the crash when executing the test suite. And using
c_str() here is unnecessary anyhow.

See #14214.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71317 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775