vhost: remove dequeue zero-copy support

vhost: remove dequeue zero-copy support

Dequeue zero-copy removal was announced in DPDK v20.08.
This feature brings constraints which makes the maintenance
of the Vhost library difficult. Its limitations makes it also
difficult to use by the applications (Tx vring starvation).

Removing it makes it easier to add new features, and also remove
some code in the hot path, which should bring a performance
improvement for the standard path.

Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Reviewed-by: Chenbo Xia <chenbo.xia@intel.com>