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