libprocessgroup: Remove CGROUPV2_CONTROLLER_NAME

This was deprecated a year ago, and should no longer be used.

Change-Id: I19c75e50fd3cc86734fa9cc7f3d883260b2484ca
diff --git a/libprocessgroup/include/processgroup/processgroup.h b/libprocessgroup/include/processgroup/processgroup.h
index 85796ea..02f7c25 100644
--- a/libprocessgroup/include/processgroup/processgroup.h
+++ b/libprocessgroup/include/processgroup/processgroup.h
@@ -27,7 +27,6 @@
 __BEGIN_DECLS
 
 static constexpr const char* CGROUPV2_HIERARCHY_NAME = "cgroup2";
-[[deprecated]] static constexpr const char* CGROUPV2_CONTROLLER_NAME = "cgroup2";
 
 bool CgroupsAvailable();
 bool CgroupGetControllerPath(const std::string& cgroup_name, std::string* path);