Remove references to legacy comment IDs

Summary:
Ref T2222. I want to stage a "later" patch to drop this column, but get rid of the last few references to it.

One of these methods has no callers, and the other stuff I've updated to use the modern fields.

Test Plan: Created some inlines, hit "edit", submitted them, `grep.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2222

Differential Revision: https://secure.phabricator.com/D8240