Delete secret from plugin only if there's metadata

If for some reason there is no metadata available for a certain
secret, the secret deletion will fail, as it tries to delete it from
the plugin first. This change makes that optional. Thus, now a secret
with no metadata will either way be deleted from the database.

Change-Id: Iab6699e300e9688f26dd39d0c947c9a45fb3384f
Closes-Bug: 1377330
Merge "Delete secret from plugin only if there's metadata"