Remove checking PolicyType in libcynara-admin

Remove checking PolicyType in libcynara-admin

Two libcynara-admin API functions:
* cynara_admin_set_policies
* cynara_admin_set_bucket
have checked validity of PolicyType. Except for special values
(like NONE, BUCKET, DELETE) they accepted only ALLOW and DENY
policy types. No other policy types defined by plugins were able
to be set.

This patch removes described checks leaving only special values
handling. All other types (including ALLOW and DENY) are passed
to cynara service.

The only check that is done verifies if given policy type can
be casted into Cynara::PolicyType.

Change-Id: Iedb0434bd35d4085e977fe86d07ed33a1f89cdc7