Remove adb_policy_init and adb_policy_close

Since f72c3ffa the policy is initialized as part of database.
adb_policy_close is now a no-op and adb_policy_init just makes
sure the database is initialized. adb_policy_init is only called
from kadm5_flush and only if database initialization was
successful beforehand rendering this call redundant.

This removes adb_policy_init and adb_policy_close definitions and
all their references in the code and doc.