RDMA/cm: Remove the cm_free_id() wrapper function

Just call xa_erase directly during cm_destroy_id()

Link: https://lore.kernel.org/r/20200506074701.9775-9-leon@kernel.org
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>