Remove adb_policy_init and adb_policy_close

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.

Remove adb_policy_init and adb_policy_close and all their references
in the code and documentation.