bus_generic_detach: Remove redundant check

device_detach() checks the device state and only calls a driver's
DEVICE_DETACH method if the device is attached but not busy.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D47156

(cherry picked from commit 3342afcbaf42c2e6a4604c84e267901411e790ca)