Move PATH checks to post-install

Summary:
We no longer need to do PHP CLI checks (D4568) or run `git submodule` (D4581) so we don't need $PATH to be set to complete setup. Move it to post-install.

Drop the instructions about PHP-FPM because the Phabricator config is dramatically easier now that we have it.

Test Plan: Set environment.append-paths to various things, faked lack of $PATH, verified I got the warning when I expected to setting Phabricator config cleared it.

Reviewers: codeblock, btrahan

Reviewed By: codeblock

CC: aran

Maniphest Tasks: T2228

Differential Revision: https://secure.phabricator.com/D4585