Remove "Create Empty Task" workflow callouts and some other clutter

Summary:
Ref T9908. This removes the "Create Another Empty Task", "Create A Similar Task" and "Create Another Subtask of Same Parent Task" workflow callouts on the task detail page, which are actions that show up after creating a task or creating a subtask.

• I think "Create Empty" isn't relevant now that we have "Create Task" nearby and the quick create menu?
• I'm not sure if "Create Similar" is worth keeping. If we do want to retain it, I'd maybe like to find a way to do it generically.
• I'm likewise not sure if "Create another subtask" is worth keeping.

Overall, these actions are weird/unusual and I'm not sure how valuable they are. I'm open to keeping "Similar" and/or "Subtask" but I'd like to verify that they're still valuable and make sure we have a reasonable design for them if we retain them.

For example, if we want to retain "Similar", maybe a better approach is just to add "Create Similar Object" to every action menu (which is now possible for EditEngine applications)? There's at least some interest in "Create Similar Repository" in Diffusion.

Also removes a very very old piece of "attached files" code.

Test Plan: Created and viewed some tasks.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9908

Differential Revision: https://secure.phabricator.com/D14705