x86: Remove dead config option X86_CPU

This isn't being referenced anywhere, and the selects done from
it can be easily done together with all the other X86 ones.

 v2: Also adjust UML's Kconfig.x86.

Signed-off-by: Jan Beulich <jbeulich@novell.com>
LKML-Reference: <4D7603DA02000078000351C1@vpn.id2.novell.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>