uml: remove fakehd

The fakehd switch lost its implementation at some point.  Since no one is
screaming for it, we might as well remove it.

Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>