Delete PhabricatorRemarkupRuleProxyImage

Summary: don't need it now that uploading files is so easy. Plus it made for some buggy jonx if / when there were bad image links coupled with caching. In theory this is a lot less pretty though if folks linked to a bunch of files served elsewhere using images.

Test Plan: http://does-not-exist.com/imaginary.jpg rendered as a link!

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2000

Differential Revision: https://secure.phabricator.com/D3908