Remove reentrant code

Summary: Ref T7604. Remove the reenter_if_this_is_arcanist_or_libphutil function. Some discussion in D12945.

Test Plan: Ran ./bin/arc --trace help and saw things happen as expected.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7604

Differential Revision: https://secure.phabricator.com/D12999