Remove accidental double-rendering of content sources from Differential

Summary: Oops, I left this in from an earlier version and missed it since I was
mostly looking at Maniphest for testing. We already render this information in
the header, don't additionally render it under the comments.

Test Plan: derp derp, loaded any revision with sourced comments

Reviewers: jungejason, tuomaspelkonen, aran

Reviewed By: jungejason

CC: aran, jungejason

Differential Revision: 871