examples/vhost: remove unused macro and struct

Interestingly, DESC_PER_CACHELINE has never been used since the
introduction of vhost example. Remove it.

vlan_ethhdr struct and VLAN_ETH_HLEN macro reference had been removed
by commit 4d50b6acbd95 ("examples/vhost: adapt Tx routing to lib"), but
had forgot to remove the definition.

Fixes: 4d50b6acbd95 ("examples/vhost: adapt Tx routing to lib")

Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>