Drivers that need a uverbs AH should instead set the create_user_ah() op
similar to reg_user_mr(). MODIFY_AH and QUERY_AH cmds were never
implemented so are just deleted.

Link: https://lore.kernel.org/r/11-v1-caa70ba3d1ab+1436e-ucmd_mask_jgg@nvidia.comSigned-off-by: Jason Gunthorpe <jgg@nvidia.com>