net/vhost: remove feature related APIs

The rte_eth_vhost_feature_disable/enable/get APIs are just a wrapper of
rte_vhost_feature_disable/enable/get. However, the later are going to
be refactored; it's going to take an extra parameter (socket_file path),
to let it be per-device.

Instead of changing those vhost-pmd APIs to adapt to the new vhost APIs,
we could simply remove them, and let vdev to serve this purpose. After
all, vdev options is better for disabling/enabling some features.

Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Acked-by: Maxime Coquelin <maxime.coquelin@redhat.com>