RDMA/cm: Remove unused cm_class

Previous commits removed all references to the /sys/class/infiniband_cm/
directory represented by the cm_class symbol. Remove the directory and
cm_class.

Fixes: a1a8e4a8 ("rdma: Delete the ib_ucm module")
Link: https://lore.kernel.org/r/0-v1-90096a98c476+205-remove_cm_leftovers_jgg@nvidia.com


Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>