RDMA/cxgb4: Remove MW support

This driver never enabled IB_USER_VERBS_CMD_ALLOC_MW so memory windows
were not usable from userspace. The kernel side was removed long ago. Drop
this dead code.

Fixes: feb7c1e3 ("IB: remove in-kernel support for memory windows")
Link: https://lore.kernel.org/r/1-v1-caa70ba3d1ab+1436e-ucmd_mask_jgg@nvidia.com


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