Make unsubscribing from a project have an effect

Summary:
Fixes T10089. This did work at one point, but was broken by D12868, which got too aggressive about mailing members.

We don't want to send mail to all members by default, only those who are subscribed. The parent implementation of getMailCC() handles this for us.

Test Plan:
Joined a project as users A and B. Unsubscribed with B. Made an edit.

Before patch: both A and B got mail. After patch: only A got mail.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10089

Differential Revision: https://secure.phabricator.com/D14955