x86_32: clean up play_dead

The removal of the CPU from the various maps was redundant as it already
happened in cpu_disable.

After cleaning this up, cpu_uninit only resets the tlb state, so rename
it and create a noop version for the X86_64 case (so the two play_deads
can be unified later).

Signed-off-by: Alex Nixon <alex.nixon@citrix.com>
Acked-by: Jeremy Fitzhardinge <jeremy@goop.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>