libprocessgroup: Stop services that have been migrated to another v2 cgroup

CL 2828279 doesn't do what it's description says that it does. Making
Service::Stop() work for processes that have been migrated to another v2
cgroup requires changing DoKillProcessGroupOnce(). Hence this CL that
removes the early return statements from DoKillProcessGroupOnce().

Bug: 308900853
Change-Id: Ib798555feeb95a786a619c3d7013c7d5829b01ad
Signed-off-by: Bart Van Assche <bvanassche@google.com>