Fix an out-of-date API call on the destruction pathway for Pholio mocks

Summary:
See https://discourse.phabricator-community.org/t/destroying-a-mock-using-bin-remove-destroy-mx-gives-an-error/3728.

Currently, Pholio calls an older API method on the mock destruction pathway. This call was introduced in D19911 but the callsite was only partially updated in D19914.

Test Plan: Ran "bin/remove destroy Mx" to destroy a mock. Before: fatal with a bad call; after: clean destruction.

Differential Revision: https://secure.phabricator.com/D21081