examples/multi_process: remove l2fwd fork example

examples/multi_process: remove l2fwd fork example

l2fwd_fork relies on a multiprocess model that DPDK does not support
(calling rte_eal_init() before fork()), in particular in light of recent
EAL changes like the multiproess communication channel.

This example can mislead users into thinking this is a supported
multiprocess model; hence, this commit removes this example and the
corresponding user guide documentation as well.

This patch was made following this mailing list discussion:
http://mails.dpdk.org/archives/dev/2018-July/108106.html

Signed-off-by: Gage Eads <gage.eads@intel.com>