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