RDMA/ucontext: Get rid of the old disassociate flow

The disassociate_ucontext function in every driver is now empty, so we
don't need this ugly and wrong code that was messing with tgids.

rdma_user_mmap_io does this same work in a better way.

Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>