mxge: cleanup from purging EOL release compatibility

The FreeBSD version was being used to trigger preprocessor conditionals
IFNET_BUF_RING and MXGE_VIRT_JUMBOS. Since those are no longer
conditional, purge their effects.

Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/603
Differential Revision: https://reviews.freebsd.org/D35560