Drop DHCP support for FreeBSD network cards

Summary:
Plan is to simplify network configuration and allow correct configuration
of the interfaces cards through Salt. As part of this, we can drop variables
not used anymore.

Mandatory DHCP connection was a think of Online to authenticate a MAC address
on the network.

Test Plan: no op

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D2570