Remove unused macro from common/drv.c.

When we was compering it to code from boot2 it also looks like
this code is buggy and boot2 was never updated to use this code.
USE_XREAD flag is unused in boot2, and common/drv.c was never
build with that flag.

Reviewed by: jhb
Differential Revision: https://reviews.freebsd.org/D9780