Remove the obsolete "DiffusionInlineCommentPreviewController"

Summary: Ref T13513. This controller was obsoleted by EditEngine and appears unreachable without explicitly typing the URL.

Test Plan:

• Grepped for the route, didn't find any hits.
• Deleted the controller, successfully previewed comments in Diffusion.

Maniphest Tasks: T13513

Differential Revision: https://secure.phabricator.com/D21224