Reset print_sequence when reactivating extruder 2

The setting print_sequence was not being resetted to all at once
whenever the 2nd extruder was reactivated.
This commit fixes that by explicitly resetting print_sequence
when reenabling an extruder, if it has been changed by the user.

CURA-6914

cura/Settings/MachineManager.py

@@ -747,6 +747,11 @@ class MachineManager(QObject):
         result = []  # type: List[str]
         for setting_instance in container.findInstances():
             setting_key = setting_instance.definition.key
+            if setting_key == "print_sequence":
+                old_value = container.getProperty(setting_key, "value")
+                Logger.log("d", "Reset setting [%s] in [%s] because its old value [%s] is no longer valid", setting_key, container, old_value)
+                result.append(setting_key)
+                continue
             if not self._global_container_stack.getProperty(setting_key, "type") in ("extruder", "optional_extruder"):
                 continue