blk-cgroup: remove pointless CONFIG_BLOCK ifdefs

No need to make BLK_CGROUP stubs conditional on CONFIG_BLOCK as they
can't be used without that.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/20220420042723.1010598-11-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>