When a paste's file is deleted, drop it from query results

Summary:
We allow files to be deleted from the web UI, including paste files. When a paste's file is deleted, we currently continue to show it in the paste list and then 400 when it's clicked on.

Instead, remove it from the list. (Note that it may stick around if it's cached.)

Test Plan: Purged general cache, deleted a paste's file, viewed paste list, saw no pastes.

Reviewers: dctrwatson, btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3265

Differential Revision: https://secure.phabricator.com/D6067