Remove phutil_exit()

Summary: Fixes T8219. This mechanism is fraught with peril (confusing to developers and users) and not particularly valuable.

Test Plan: Grepped for everything I removed. Loaded some pages.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T8219

Differential Revision: https://secure.phabricator.com/D12885