DPDK: Remove support for vhost-user zero-copy.

DPDK: Remove support for vhost-user zero-copy.

Support for vhost-user dequeue zero-copy was deprecated in OVS 2.14 with
the aim of removing it for OVS 2.15.

OVS only supports zero copy for vhost client mode, as such it will cease
to function due to DPDK commit [1]

Also DPDK is set to remove zero-copy functionality in DPDK 20.11 as
referenced by commit [2]

As such remove support from OVS.

[1] 715070ea10e6 ("vhost: prevent zero-copy with incompatible client mode")
[2] d21003c9dafa ("doc: announce removal of vhost zero-copy dequeue")

Signed-off-by: Ian Stokes <ian.stokes@intel.com>
Acked-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Acked-by: Ilya Maximets <i.maximets@ovn.org>
Acked-by: Kevin Traynor <ktraynor@redhat.com>