vxge(4): deprecation notice

This hardware isn't totally ancient, about equal to a mxge(4) or mlx4en(4),
but the company was sold to Exar which then promptly exited the Ethernet
business so the card was commercially available for under 2 years. On deep
search, the only usage of these cards I found was by the importing of the
driver. There are code quality issues identified by Brooks and Hiren and
no visible use nor maintainership that warrant removal from FreeBSD 12.0.

Submitted by: kbowling
Reviewed by: gnn brooks
Sponsored by: Limelight Networks
Differential Revision: https://reviews.freebsd.org/D15363