Remove registry_file variable in filebeat-7.yml (#92)

Beats version 7 has removed filebeat.registry_file setting [1]. To reflect this
change, registry_file variable is removed from filebeat-7.yml file to support
filebeat version 7.

[1] https://www.elastic.co/guide/en/beats/libbeat/current/release-notes-7.0.0.html

templates/filebeat-7.yml

@@ -1,7 +1,6 @@
 # WARNING! This file is managed by Juju. Edits will not persist.
 # Edit at your own risk
 filebeat:
-  registry_file: /var/lib/filebeat/registry
   inputs:
     - type: log
       paths: