Maniphest - remove "attach file" action

Summary: Fixes T4655. Basically leaves the display code intact for legacy installs but removes the option from the UI and removes "create" code.

Test Plan:
tried to attach file and the action was not in the dropdown!
made a new task and it worked!
commented on an old task and it worked!

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T4655

Differential Revision: https://secure.phabricator.com/D8777