klippy: Convert printer_state("shutdown") to an event handler

Convert all users of the printer_state("shutdown") handler to register
a "klippy:shutdown" event handler instead.

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

klippy/mcu.py

@@ -422,6 +422,7 @@ class MCU:
         self._name = config.get_name()
         if self._name.startswith('mcu '):
             self._name = self._name[4:]
+        self._printer.register_event_handler("klippy:shutdown", self._shutdown)
         # Serial port
         self._serialport = config.get('serial', '/dev/ttyS0')
         baud = 0
@@ -769,8 +770,6 @@ class MCU:
             self._connect()
         elif state == 'disconnect':
             self._disconnect()
-        elif state == 'shutdown':
-            self._shutdown()
     def __del__(self):
         self._disconnect()