MAINTAINERS: Remove Jeremy from the Xen subsystem.

Jeremy has been a key person in making Linux work with Xen.
He has been enjoying the last year working on something
different so reflect that in the maintainers file.

CC: Jeremy Fitzhardinge <jeremy@goop.org>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: Jeremy Fitzhardinge <jeremy@goop.org>