powerpc/xive: Remove P9 DD1 flag XIVE_IRQ_FLAG_EOI_FW

This flag was used to support the P9 DD1 and we have stopped
supporting this CPU when DD2 came out. See skiboot commit:

  https://github.com/open-power/skiboot/commit/0b0d15e3c170



Also, remove eoi handler which is now unused.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Greg Kurz <groug@kaod.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201210171450.1933725-11-clg@kaod.org