x86/apic: Drop enum apic_delivery_modes

The type is not used any more.

Replace the constants with plain defines so they can live outside of an
__ASSEMBLY__ block, allowing for more cleanup in subsequent changes.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Steve Wahl <steve.wahl@hpe.com>
Link: https://lore.kernel.org/r/20231102-x86-apic-v1-2-bf049a2a0ed6@citrix.com