Remove 'Images' from editing a mock

Summary: For now, remove the images field when editing a mock, can add it back if we decide to allow it.

Test Plan: Check edit mock, field gone, check new mock, field is alive and well.

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2552

Differential Revision: https://secure.phabricator.com/D5289