Get rid of overridden wxTextCtrl::DoSetValue() in wxGTK

This method seems completely unnecessary, the base
wxTextEntry::DoSetValue(), which delegates to Remove() and WriteText(),
seems to work just as well and avoids code duplication between this
method and wxTextCtrl::WriteText().

Notice that gtk_text_buffer_set_text() is just a trivial wrapper around
gtk_text_buffer_delete() and gtk_text_buffer_insert() anyhow, so there
is no efficiency loss in not using it neither.

include/wx/gtk/textctrl.h

@@ -158,7 +158,6 @@ protected:
     // override this and return true.
     virtual bool UseGTKStyleBase() const wxOVERRIDE { return true; }
 
-    virtual void DoSetValue(const wxString &value, int flags = 0) wxOVERRIDE;
     virtual wxString DoGetValue() const wxOVERRIDE;
 
     // Override this to use either GtkEntry or GtkTextView IME depending on the