Revert "libprocessgroup: Poll on cgroup.events"

This revert was created by Android Culprit Assistant. The culprit was identified in the following culprit search session (http://go/aca-get/91da3c52-9b76-498b-bdbd-a9de7d7ff53b).

Change-Id: I996c595bee9acc15aedaf0a912f67fa027f33cd0