MAINTAINERS: Remove myself from XIVE

MAINTAINERS: Remove myself from XIVE

Working on XIVE has been one of the most complex and fascinating
experiences for me. It's been a real journey, and now it's time for
IBM to take over and guide its future. I'm stepping back as the
maintainer of XIVE.

Cc: Michael Kowal <kowal@linux.ibm.com>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Frédéric Barrat <fbarrat@linux.ibm.com>
Signed-off-by: Cédric Le Goater <clg@redhat.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>