inherit assembly plugin version from parent

git-svn-id: https://svn.apache.org/repos/asf/maven/enforcer/trunk@1465379 13f79535-47bb-0310-9956-ffa450edef68
remove cobertura clean

git-svn-id: https://svn.apache.org/repos/asf/maven/enforcer/trunk@1465380 13f79535-47bb-0310-9956-ffa450edef68