[identity] make sure otk exists when we delete it

Summary: since DeleteItem is idempotent, we need to check that the key actually exists when we try deleting it, otherwise we'll be decrementing the otk count but not actually removing any otks

Test Plan: confirmed that if the key doesn't exist, the transaction fails

Reviewers: bartek

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11678