Kill PhabricatorFileURI

Summary:
we used to need this function for security purposes, but no longer need
it. remove it so that some call sites can be optimized via smarter data
fetching, and so the whole codebase can have one less thing in it.

Test Plan:
verified the images displayed properly for each of the following

• viewed a diff with added images.
• viewed a user feed
• viewed a user profile
• viewed all image macros
• viewed a paste and clicked through "raw link"

weakness in testing around proxy files and transformed files. not sure what
these are. changes here are very programmatic however.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, btrahan, epriestley

Maniphest Tasks: T672

Differential Revision: https://secure.phabricator.com/D1354