memcg: remove cgroup_event->cft

memcg: remove cgroup_event->cft

The only use of cgroup_event->cft is distinguishing "usage_in_bytes"
and "memsw.usgae_in_bytes" for mem_cgroup_usage_[un]register_event(),
which can be done by adding an explicit argument to the function and
implementing two wrappers so that the two cases can be distinguished
from the function alone.

Remove cgroup_event->cft and the related code including
[un]register_events() methods.

Signed-off-by: Tejun Heo <[email protected]>
Acked-by: Li Zefan <[email protected]>
Acked-by: Kirill A. Shutemov <[email protected]>
Acked-by: Michal Hocko <[email protected]>