RDMA/mlx5: Remove implicit ODP cache entry

Implicit ODP mkey doesn't have unique properties. It shares the same
properties as the order 18 cache entry. There is no need to devote a
special entry for that.

Link: https://lore.kernel.org/r/20230125222807.6921-3-michaelgur@nvidia.com
Signed-off-by: Aharon Landau <aharonl@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>