doc: Remove ivshmem instructions.

doc: Remove ivshmem instructions.

ivshmem is a path to the guest using DPDK rings that was
introduced before userspace vhost was available in the OVS-DPDK
datapath. ivshmem is external to OVS but the scheme of using it
with DPDK rings is documented.

Remove ivshmem instruction documentation because:

- The ivshmem library has been removed in DPDK since DPDK 16.11.
- The instructions/scheme provided will not work with current
  supported and future DPDK versions.
- The linked patch needed to enable support in QEMU has never
  been upstreamed and does not apply to the last 4 QEMU releases.
- Userspace vhost has become the defacto OVS-DPDK path to the guest.

Fixes: 04de404e1bfa ("netdev-dpdk: Add support for DPDK 16.11")
Cc: Ciara Loftus <ciara.loftus@intel.com>
Cc: Stephen Finucane <stephen@that.guru>
Signed-off-by: Kevin Traynor <ktraynor@redhat.com>
Acked-by: Stephen Finucane <stephen@that.guru>
Acked-by: Mauricio Vasquez B <mauricio.vasquez@polito.it>
Signed-off-by: Daniele Di Proietto <diproiettod@vmware.com>