Merge wxComboCtrl::InstallInputHandlers() into CreateTextCtrl()

This function was always called after calling CreateTextCtrl() and
couldn't be called at any other moment, so it didn't make much sense to
have it as a separate function, just install the custom input handler
when (re)creating the text control.

This simplifies the derived classes code.

include/wx/combo.h

@@ -509,9 +509,6 @@ protected:
     // (e.g. from WriteText())
     void OnSetValue(const wxString& value);
 
-    // Installs standard input handler to combo (and optionally to the textctrl)
-    void InstallInputHandlers();
-
     // Flags for DrawButton
     enum
     {