x86/apic: Drop apic::delivery_mode

This field is set to APIC_DELIVERY_MODE_FIXED in all cases, and is read
exactly once.  Fold the constant in uv_program_mmr() and drop the field.

Searching for the origin of the stale HyperV comment reveals commit
a31e58e1 ("x86/apic: Switch all APICs to Fixed delivery mode") which
notes:

  As a consequence of this change, the apic::irq_delivery_mode field is
  now pointless, but this needs to be cleaned up in a separate patch.

6 years is long enough for this technical debt to have survived.

  [ bp: Fold in
    https://lore.kernel.org/r/20231121123034.1442059-1-andrew.cooper3@citrix.com


  ]

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-v1-1-bf049a2a0ed6@citrix.com