Fix leak in kadm5_flush with LDAP KDB

Due to an inverted test in adb_policy_init, kadm5_flush calls
krb5_db_open twice.  With the DB2 KDB module, the second open is a
no-op, but with the LDAP module, a new DB handle is allocated and the
old one is leaked.

[ghudson@mit.edu: rewrote commit message]

ticket: 7897 (new)
target_version: 1.12.2
tags: pullup
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.