usb: purge EOL release compatibility
Remove conditionsals for FreeBSD 6, 7, 8 and 9 compatibility.
Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/603
Differential Revision: https://reviews.freebsd.org/D35560
(cherry picked from commit 336fbb23def7a6b3dc44ce5c76bbc07a701c0a69)