hptmv: remove vestigial EVENTHANDLER_DEREGISTER()

The registration was removed in favor of DEVICE_SHUTDOWN(). Drop the
unused eventhandler tag from the IAL_ADAPTER_T structure.

Reviewed by: jhb
Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Fixes: cd3ef66680a8 ("Use DEVICE_SHUTDOWN(9) mechanism...")
Differential Revision: https://reviews.freebsd.org/D42334