cgroup: make cgroup_is_removed() static

cgroup_is_removed() no longer has external users and it shouldn't grow
any - controllers should deal with cgroup_subsys_state on/offline
state instead of cgroup removal state.  Make it static.

While at it, make it return bool.

Signed-off-by: Tejun Heo <tj@kernel.org>

diff --cc include/linux/cgroup.h
Simple merge

diff --cc kernel/cgroup.c
Simple merge