Remove the ArcanistBaseWorkflow::getUserGUID function.

Summary: This function has been deprecated for a long time (see rARC8150fdf044818c503a588c1b3ef0ddbb93cfa1be). It should be safe to remove it now.

Test Plan: N/A

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9465