MAC/do: Remove the 'prison0' special cases in the common paths

The rules on 'prison0' are initialized in init(), now using
set_empty_rules().

Until the jail is destroyed, they can never be uninitialized by a call
to osd_jail_del(), since the only chain to call it is
mac_do_prison_set() -> remove_rules() -> osd_jail_del(), and
mac_do_prison_set() (method PR_METHOD_SET) can never be called on
'prison0'. This guarantees that find_rules() always find a valid
'rules' pointer to return.

There's no need to do anything special in destroy() for 'prison0', as
osd_jail_deregister() now takes care of it.

Reviewed by: bapt
Approved by: markj (mentor)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D47603