ansible.cfg: Unset vault_password_file

Apparently, the `vault_password_file` setting in `ansible.cfg` overrides
the `--vault-password-file` command-line argument, which breaks the
Jenkins `ansiblePlaybook` task.

ansible.cfg

@@ -7,6 +7,4 @@ gathering = smart
 fact_caching = jsonfile
 fact_caching_connection = .fact-cache
 
-vault_password_file = .vault-secret.sh
-
 remote_tmp = /var/tmp/.ansible-${USER}