MAINTAINERS: Remove iommu@lists.linux-foundation.org

The IOMMU mailing list has moved to iommu@lists.linux.dev
and the old list should bounce by now. Remove it from the
MAINTAINERS file.

Cc: stable@vger.kernel.org
Signed-off-by: Joerg Roedel <jroedel@suse.de>
Link: https://lore.kernel.org/r/20220706103331.10215-1-joro@8bytes.org