Remove all "FileHasObject" edge reads and writes

Summary: Ref T13603. Removes all reads and writes to the other half of the legacy edge.

Test Plan:

• Verified I could still see file content accessible only through object attachment.
• This fixes a unit test broken by removal of only half the edge access in the previous change.

Maniphest Tasks: T13603

Differential Revision: https://secure.phabricator.com/D21820