Audit - delete duplicate audit requests and add unique key

Summary: Fixes T1768. This is mostly a data cleanliness issue as duplicate rows don't really do anything, but let's clear it up now.

Test Plan: made some duplicate rows by adding the same auditor multiple times. ran ./bin/storage upgrade and it worked perfectly!

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T1768

Differential Revision: https://secure.phabricator.com/D10849