Revert "block: add __init to blkcg_policy_register"

This reverts commit a2d445d440003f2d70ee4cd4970ea82ace616fee.

The original commit is buggy, we do use the registration functions
at runtime for modular builds.

diff --cc block/blk-cgroup.c
Simple merge

diff --cc block/blk-cgroup.h
Simple merge