Clean up some old scripts in libphutil

Summary:
  - `envinfo.php` has been obsoleted by:
    - `__init_script__.php` just sets all this stuff to the right value now.
    - `support/debug.php` provides a more general mechanism to get arbitrary environmental data from users.
  - `kill_init.php` facilitated the removal of `__init__.php` files, but we did that a long time ago and I haven't seen any users run into issues in many months.

Test Plan: N/A

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D6377