For unknown reasons, setting queryForm: here prevents loading
the option list in the combobox.
However: the queryForm parameter only would only make sense if we also
set 'editable' to 'true', since queryForm specifies how the drop down
list should react to user input in the text field of the combobox.
Hence it is safe to remove it.

Tested with ExtJS4 and ExtJS5, this patch has no impact on
the rendering of the underneath node tree.