Remove all current Authors / Reviewers from "CCWelcome" mail

Summary:
We'll incorrectly send CCWelcome mail to users who would be added as CCs but are blocked by the new "$dont_add" stuff, for
example when a revision is updated and the user has a Herald rule which triggers them getting CC'd. See D2057.

Potentially a better fix for this would be to have "addCCs" return a list of the CCs it actually added, rather than duplicating the
logic of removing CCs in two places. However, that's not trivial since it's just a wrapper around alterRelationships() which is nasty
and would need a more complicated return type. I think this whole thing will get a refactoring pass at some point -- I want to build a
more generic "associations"-like datastore and replace some of the ad-hoc associations with it. So maybe I can clean it up when that
happens. For now, this should fix the immediate problem.

Test Plan: Updated a revision, didn't get CC welcomed.

Reviewers: vrana, btrahan

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D2072