Remove unused methods in PhabricatorEditorExtension

Summary:
The methods newRequiredTransasctionError() [sic!] and newInvalidTransactionError() are not called anywhere in the codebase. Both are final protected, thus remove them.

Incidentally, speedup Diviner generation by at least 0.000000000000000000000001 milliseconds.

Test Plan: Grep the code.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25643