Remove pcntl/php CLI setup checks

Summary:
As far as I know, we never actually need `php` to be available from the web UI. I think the history here is:

  - Long ago, we checked for 'pcntl' as an extension during setup.
  - Someone had an install where 'pcntl' was available from the CLI, but not the web UI. So we switched the check to use the CLI.
  - Someone had an install where the CLI binary was php-fpm, which caused the 'pcntl' check to loop endlessly, so we added more checks.

But we don't actually need to do any of this -- when the user tries to run the daemons, they get an explicit message that they need to install pcntl already, and we never (as far as I know) try to run PHP scripts from the web UI other than the pcntl_available.php check (we only run `git`, `svn`, `hg`, `ssh-agent`, `diff`, `xhpast` and `pygmentize`, I think).

Test Plan: Thought carefully about places we might execute PHP scripts from the web UI. Looked through /scripts/ to try to identfiy anything we might execute.

Reviewers: btrahan, vrana

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D4568