Remove imagePHIDs column from ConpherenceThread

Summary: Ref T11730. Removes the unused column, seen no issues during past week migrations.

Test Plan: Run migration, check database no longer contains column.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T11730

Differential Revision: https://secure.phabricator.com/D16711