Remove obsolete "setDisableMacros()" on "PhabricatorRemarkupControl"

Summary:
Ref T13682. This method originated in D4781 and no longer has any callers; remove it.

(I believe this was from an era where macros didn't have to be on their own line to trigger.)

Test Plan: Grepped for method, found no callers.

Maniphest Tasks: T13682

Differential Revision: https://secure.phabricator.com/D21831