Remove daemon PID files for missing daemons when running "phd stop"

Summary: When we try to kill a daemon but discover it isn't running, we should
remove the PID file. We can also simplify the logic here.

Test Plan: Ran "phd stop" a couple of times, subsequent runs did not try to stop
a legion of dead daemons.

Reviewers: btrahan, jungejason

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T781

Differential Revision: https://secure.phabricator.com/D1421