Remove obsolete write to "pid" property in "annihilateProcessGroup()" in Daemon Overseer

Summary: Ref T13579. This property was removed in D21425, but I missed this usage site. Remove the assignment; this class no longer tracks the subprocess PID directly.

Test Plan: Searched for "->pid", no further hits.

Maniphest Tasks: T13579

Differential Revision: https://secure.phabricator.com/D21452

src/infrastructure/daemon/PhutilDaemonHandle.php

@@ -356,7 +356,6 @@ final class PhutilDaemonHandle extends Phobject {
         posix_kill(-$pgid, SIGTERM);
         sleep($this->getKillDelay());
         posix_kill(-$pgid, SIGKILL);
-        $this->pid = null;
       }
     }
   }