Remove unassigned_access CPU hook

Remove unassigned_access CPU hook

All targets have now migrated away from the old unassigned_access
hook to the new do_transaction_failed hook. This means we can remove
the core-code infrastructure for that hook and the code that calls it.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20191108173732.11816-1-peter.maydell@linaro.org