MAINTAINERS: update block/sheepdog maintainers

E-mail to one of block/sheepdog maintainers Mitake Hitoshi bounces

<mitake.hitoshi@lab.ntt.co.jp>: unknown user: "mitake.hitoshi"

and no current address is known. So just remove it.
Signed-off-by: NLiu Yuan <liuyuan1@cmiot.chinamobile.com>
Reviewed-by: NMarkus Armbruster <armbru@redhat.com>
Signed-off-by: NThomas Huth <thuth@redhat.com>

MAINTAINERS

@@ -2023,7 +2023,6 @@ F: block/rbd.c
 T: git git://github.com/codyprime/qemu-kvm-jtc.git block
 
 Sheepdog
-M: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
 M: Liu Yuan <namei.unix@gmail.com>
 M: Jeff Cody <jcody@redhat.com>
 L: qemu-block@nongnu.org