restapi: Fix request body type definition for delete schedulingpolicyunits

delete schedulingpolicyunits command have no body, so its type should
be defined as 'null'

Change-Id: I1b27651773c8a4d3d6bfd0bb9af9f7cd5d26154e
Bug-Url: https://bugzilla.redhat.com/1113484
Signed-off-by: Yevgeny Zaspitsky <yzaspits@redhat.com>