Avoid GTK warnings when calling Clear() on a wxBitmapComboBox

Use wxTextEntry::Clear(), now that it uses Remove() instead of SetValue().
See #16654 and fc8d3f6

src/gtk/combobox.cpp

@@ -297,12 +297,7 @@ wxComboBox::GetClassDefaultAttributes(wxWindowVariant WXUNUSED(variant))
 
 void wxComboBox::Clear()
 {
-    // Do not call wxTextEntry::Clear() here as it's implemented in terms of
-    // virtual SetValue() and so would call our own overridden version of this
-    // method, which wouldn't do the right thing in wxCB_READONLY case.
-    //
-    // Clear the text directly to avoid this.
-    wxTextEntry::SetValue(wxString());
+    wxTextEntry::Clear();
     wxItemContainer::Clear();
 }