MAINTAINERS: Remove myself as maintainer

I am leaving IBM and will move on to other working area,
so remove myself as a vfio-ccw maintainer.

Signed-off-by: Dong Jia Shi <bjsdjshi@linux.ibm.com>

Message-Id: <20180706015743.41810-1-bjsdjshi@linux.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>