bhnd: Remove orphaned driver for the BHND USB core

This was only connected to the build for MIPS and has been
disconnected from the build since MIPS was removed.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D45432

(cherry picked from commit c7c037fad1862c18e3e9635b06194a4ca00ac3c1)