MAINTAINERS: Drop Frank Rowand from DT maintainership

Frank retired a while back and surprisingly has found better things to
do than DT maintainership. Thank you Frank for all of your help with DT
for many years.

Link: https://lore.kernel.org/r/20240122211501.1719374-1-robh@kernel.orgSigned-off-by: Rob Herring <robh@kernel.org>

MAINTAINERS

@@ -16359,7 +16359,6 @@ F:	drivers/infiniband/ulp/opa_vnic
 
 OPEN FIRMWARE AND FLATTENED DEVICE TREE
 M:	Rob Herring <robh+dt@kernel.org>
-M:	Frank Rowand <frowand.list@gmail.com>
 L:	devicetree@vger.kernel.org
 S:	Maintained
 W:	http://www.devicetree.org/