Cleanup nuxeo-target-platforms MP

Various cleanup is needed:

• There's a duplicate package.xml file; one has visibility PUBLIC, the other has PRIVATE.
• Target platforms: {cap-7.4-SNAPSHOT,cap-7.4-SNAPSHOT-SNAPSHOT,cap-7.4-SNAPSHOT-HF*,cap-7.4-SNAPSHOT-HF*-SNAPSHOT}
• <target name="build" description="Build Nuxeo Jenkins Report Marketplace package">
• artifact:resolveFile => artifact:file
• nuxeo.targetplatforms.version => maven.project.version

In the mean time, http://qa.nuxeo.org/jenkins/job/addons_nuxeo-target-platforms-master (duplicated by http://qa.nuxeo.org/jenkins/job/utils-nuxeo-target-platforms/) must upload the MP.