Ignore keys with trailing index on table primary key for now

Summary:
Ref T1191. We have several keys on <x, y, id>. When id is an auto-increment primary key, I believe this is exactly equivalent to a key on <x, y>, because the leaf nodes are implicitly sorted by id. We omit the implicit id elsewhere.

It would be nice to drop the id bit for consistency, but it's not doing any harm and this doesn't need to block the primary work of T1191.

Test Plan: Saw slightly fewer warnings.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T1191

Differential Revision: https://secure.phabricator.com/D10592