update_verifier: don't pass an unused command-line argument

update_verifier is being passed the current stage of Full Disk
Encryption (FDE): "nonencrypted" or "${vold.decrypt}".  However, since
FDE is no longer supported, it's actually now always "nonencrypted"
(which is confusing because there *is* encryption, it's just not FDE).
Also, update_verifier doesn't actually do anything with this
command-line argument, and based on the git history it never has.

Therefore, let's update the service definition to just have an
"update_verifier" service that passes no command-line argument.

Bug: 208476087
Test: presubmit
Change-Id: I8b62290360401024b6888c2e9488207d878d3f67