Plug memory leak with PhutilErrorTrap

Summary:
Since PhutilErrorTrap registers itself with PhutilErrorHandler, it still has a positive refcount after `unset()` and is not destroyed. Destroy it explicitly.

(I didn't test this well in the original revision. :/)

Test Plan: Ran `phd debug phabricatorbot <config> -- --trace-memory` and saw climbing memory usage before this patch. After this patch, saw steady usage.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran, staticshock

Differential Revision: https://secure.phabricator.com/D6788