cpu: Delete the empty, default off "nocpu".

cpu: Delete the empty, default off "nocpu".

This very nearly empty CPU directory does nothing except define a CPU at
the scons level which is off by default. It seems safe to delete it.

Change-Id: I5bd3a52062bbc67fb1ee7d350519cd85a3f8fdb8
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/40871
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>