R1:d31accf0768d

Add a flag.delete Conduit method

Summary: Fairly straightforward: allow deletion of flags from Conduit.

Test Plan: Try arc call-conduit with different ids.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T1556

Differential Revision: https://secure.phabricator.com/D3132