'Delete the trace callback for a variable.
MODE is one of "r", "w", "u" for read, write, undefine.
CBNAME is the name of the callback returned from trace_variable or trace.'

def trace_vdelete(self, mode, cbname):

    self._tk.call('trace', 'vdelete', self._name, mode, cbname)
    self._master.deletecommand(cbname)
