R1:84ea5c53e4e4

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…