Use wxUILocale in wxSpinCtrlDouble

This ensures that we use the decimal separator corresponding to the UI
locale of the application, rather than C locale, so that e.g. comma is
used under macOS even if setlocale() is not used.

include/wx/generic/spinctlg.h

@@ -427,8 +427,6 @@ private:
     void DoSetDigitsAndUpdate(unsigned digits);
 
 
-    wxString m_format;
-
     wxDECLARE_DYNAMIC_CLASS(wxSpinCtrlDouble);
 };