mcu: Don't define a __del__() method

The existence of a __del__() method prevents deallocation on python2
if there are circular references.  Just remove the method as
disconnect() should already be called directly.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

klippy/mcu.py

@@ -766,8 +766,6 @@ class MCU:
             self._mcu_tick_stddev)
         return False, ' '.join([msg, self._serial.stats(eventtime),
                                 self._clocksync.stats(eventtime)])
-    def __del__(self):
-        self._disconnect()
 
 Common_MCU_errors = {
     ("Timer too close", "No next step", "Missed scheduling of next "): """