libprocessgroup: Remove unused CgroupMap::SetupCgroups declaration

SetupCgroups was moved from CgroupMap in libprocessgroup to SetupCgroup
in libprocessgroup_setup, but the old declaration was not removed.

Fixes: 6f9ce2e5 ("libprocessgroup: Move CgroupSetupCgroups() to libprocessgroup_setup")
Test: m
Change-Id: I9f85000392b59a105bb202fae277a5582449822a