x86/Kconfig: Remove obsolete config X86_32_SMP

Commit

  0f08c3b2 ("x86/smp: Reduce code duplication")

removed the only use of CONFIG_X86_32_SMP.

Remove the now obsolete config X86_32_SMP too.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20231128090016.29676-1-lukas.bulwahn@gmail.com