Stop sending Phriction edit mail to the previous/current content author

Summary:
See https://discourse.phabricator-community.org/t/previous-author-of-phriction-page-always-receives-mail-when-new-author-edits-the-page/1155.

After the "send users email when they are removed from the reviewer list" change which includes old To/Cc on the list, Phriction now emails the previous editor.

This is weird and unintended. The behavior isn't necessarily always bad, but if it's sometimes desirable we should make it explicit.

For now, just get rid of the weird side effect.

Test Plan: Edited a Phriction page.

Differential Revision: https://secure.phabricator.com/D19147