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>