MAINTAINERS: Remove Keith from NVDIMM maintainers

I no longer work in this capacity for the NVDIMM or DAX subsystems.

Signed-off-by: Keith Busch <kbusch@kernel.org>
Link: https://lore.kernel.org/r/20191122162644.27078-1-kbusch@kernel.org


Signed-off-by: Dan Williams <dan.j.williams@intel.com>