base, sim, mem, arch: Remove the dummy CPU in NULL

base, sim, mem, arch: Remove the dummy CPU in NULL

The NULL ISA target has a dummy BaseCPU class that doesn't seem to be
needed anymore. Remove this class and the some unnecessary includes.

Change-Id: I031c999b3c0bb8dec036ad087a3edb2c1c723501
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/34236
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Gabe Black <gabeblack@google.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Tested-by: kokoro <noreply+kokoro@google.com>