Remove WipeDevice method from OS Agent DBus mock (#5744)

WipeDevice method was dropped from OS Agent code in [1]. Remove it from
the mock class to sync with the current API. There is no usage of
WipeDevice in the Supervisor codebase, only ScheduleWipeDevice is
called.

[1] https://github.com/home-assistant/os-agent/pull/225

tests/dbus_service_mocks/agent_system.py

@@ -28,8 +28,3 @@ class System(DBusServiceMock):
         if isinstance(self.response_schedule_wipe_device, DBusError):
             raise self.response_schedule_wipe_device  # pylint: disable=raising-bad-type
         return self.response_schedule_wipe_device
-
-    @dbus_method()
-    def WipeDevice(self) -> "b":
-        """Wipe device."""
-        return True