Fix /Private/IT-release-on-demand-packages/ job

1) nuxeo@fix-NXP-23257-release-private-packages
https://github.com/nuxeo/integration-scripts/blob/master/Jenkinsfiles/IT-release-on-demand-packages.groovy still uses `@Library('nuxeo@fix-NXP-23257-release-private-packages')`
NXP-23257 (renamed to RELEASE-402) is resolved, so the corresponding working branch is expected to be deleted soon;
maybe time to use master or a release tag? or to migrate to another feature branch which JIRA ticket is open; if waiting for rewrite of the ReleaseBuild Groovy stuff?

2) description
header and description are missing in IT-release-on-demand-packages.groovy

3) Copy artifact references
those two references are wrong since the job has been moved from /Deploy to /Private on 2017-12-01 => fix or remove

jobs/Deploy/jobs/IT-nuxeo-9.10-build/config.xml: <string>/Deploy/IT-release-on-demand-packages</string>
jobs/Deploy/jobs/IT-nuxeo-master-build/config.xml: <string>/Deploy/IT-release-on-demand-packages</string>