Remove legacy transaction editor "getDisableEmail()" method

Summary:
Ref T13042. This is a very, very old policy-violating option from yesteryear which supported build systems publishing updates by adding comments to revisions, without sending email about it.

Harbormaster has served this role for a long time and this is policy-violating in the general case (it allows attackers to act in secret).

Test Plan: Grepped for affected symbols.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13042

Differential Revision: https://secure.phabricator.com/D18881