delete upload-stemcell task as it is deprecated
delete upload-stemcell task as it is deprecated

* this was done independent of any story

Co-authored-by: Clay Kauzlaric <b161dd152ba17987305414195d30c2af856d229f@pivotal.io>

cloudfoundry-incubator/cf-routing-release,cloudfoundry-incubator/cf-routing-release,cloudfoundry-incubator/cf-routing-release