Remove "diffusion.createcomment" Conduit API method

Summary: Ref T10978. This was introduced in D6923 in 2013 as a deprecated method (before methods were extensible) and has only ever been deprecated. It no longer works after D17250 (despite my mistaken claim there that we never had an API for actions), and has been superceded by diffusion.commit.edit which is a modern, fully-power method.

Test Plan: Viewed Conduit console, no longer saw method.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10978

Differential Revision: https://secure.phabricator.com/D17254