libprocessgroup: Remove max_processes from KillProcessGroup API

The max_processes calculation is incorrect for KillProcessGroup because
the set of processes in cgroup.procs can differ between the multiple
reads in the implementation. Luckily the exact value isn't very
important because it's just logged. Remove max_processes from the API
and remove the warning about the new behavior in Android 11.

Note that we still always LOG(INFO) that any cgroup is being killed.

Bug: 301871933
Change-Id: I8e449f5089d4a48dbc1797b6d979539e87026f43