x86/kconfig: Drop unused and needless config X86_64_SMP

As part of

  38a4968b ("x86/percpu/64: Remove INIT_PER_CPU macros"),

the only use of the config option X86_64_SMP in the kernel tree is removed. As
this config option X86_64_SMP is just equivalent to X86_64 && SMP, the source
code in the tree just uses that expression in the few places where needed. Note
further that this option cannot be explicitly enabled or disabled when
configuring the kernel build configuration.

Drop this needless and unused config option. No functional change.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@redhat.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/20250723071211.622802-1-lukas.bulwahn@redhat.com