libprocessgroup: remove hardcoded freezer path

this removes the (temporary) hard-coded path to a cgroup.freeze file
to be used for checking the presence of the cgroup v2 uid/pid hierarchy
for the app freezer. From now on, only the new API must be used (see the
implementation under CacheAppOptimizer).

Bug: 180056337
Test: built without errors
Change-Id: Ifd0bce81a540a86c0c74d3e52fbe88fb731a02e1