Remove file attachment edge types, including API access

Summary:
Ref T13603. This removes the file attachment edge types, which no longer have readers or writers.

It does not delete the underlying data.

This indirectly removes API access to this edge. As far as I know, this was only used by one customer (see D21480) who has migrated away from Phabricator.

Test Plan: Grepped for edge constants.

Maniphest Tasks: T13603

Differential Revision: https://secure.phabricator.com/D21823