Remove "Used By" from Passphrase

Summary: Fixes T10972. Nothing actually updates this anymore, and only repositories ever did (e.g., Harbormaster and Drydock have never tracked it). Keeping track of this is more trouble than it's worth.

Test Plan: Grepped for constants, viewed a passphrase credential.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10972

Differential Revision: https://secure.phabricator.com/D15932