Remove wxComboBoxExtraInputHandler and use Bind() instead

There is no need to use a separate wxEvtHandler here when we can just
use Bind() to directly handle the child text control events in
wxComboCtrlBase itself, this code was a left over from pre-wx3 days.

No real changes, just make the code simpler and smaller (and also
marginally more efficient at run-time).

include/wx/combo.h

@@ -572,6 +572,9 @@ protected:
     void OnKeyEvent(wxKeyEvent& event);
     void OnCharEvent(wxKeyEvent& event);
 
+    void OnTextFocus(wxFocusEvent& event);
+    void OnTextKey(wxKeyEvent& event);
+
     // Set customization flags (directs how wxComboCtrlBase helpers behave)
     void Customize( wxUint32 flags ) { m_iFlags |= flags; }
 
@@ -627,9 +630,6 @@ protected:
     // popup interface
     wxComboPopup*           m_popupInterface;
 
-    // this is input etc. handler for the text control
-    wxEvtHandler*           m_textEvtHandler;
-
     // this is for the top level window
     wxEvtHandler*           m_toplevEvtHandler;