Remove ORDER_PATH_MODIFIED from Differential

Summary:
Ref T7803. This is a performance hack, not a real order, and isn't really meaningful or pageable.

After D12158, we constraint his query on dateModified anyway, which should generally give the database a relatively small result set to examine.

Test Plan: Browsed Differential and Diffusion. Checked query plan, it didn't look too crazy.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7803

Differential Revision: https://secure.phabricator.com/D12361