Remove rge driver from Broadcom XLR

nlge(4) is supposed to deprecate rge(4) for Broadcom XLR when it was
introduced 5 years ago.

rge doesn't build on -CURRENT due to MII changes. All the XLR kernel confs
use nlge. Let's get rid of the old driver for FreeBSD 11. We can use
10-STABLE or SVN to go back and look at the old driver if needed.

Differential Revision: https://reviews.freebsd.org/D3339
Submitted by: kevin.bowling@kev009.com