[Misc] Fixed following warning + removed dependency declaration duplication

 [WARNING] Some problems were encountered while building the effective model for org.xwiki.commons:xwiki-commons-classloader:pom:5.2-SNAPSHOT
 [WARNING] 'build.plugins.plugin.(groupId:artifactId)' must be unique but found duplicate declaration of plugin org.apache.maven.plugins:maven-enforcer-plugin @ org.xwiki.commons:xwiki-commons-core:5.2-SNAPSHOT, /Users/vmassol/dev/xwiki/git/xwiki-commons/xwiki-commons-core/pom.xml, line 150, column 15
 [WARNING]
 [WARNING] Some problems were encountered while building the effective model for org.xwiki.commons:xwiki-commons-core:pom:5.2-SNAPSHOT
 [WARNING] 'build.plugins.plugin.(groupId:artifactId)' must be unique but found duplicate declaration of plugin org.apache.maven.plugins:maven-enforcer-plugin @ line 150, column 15
 [WARNING]
 [WARNING] It is highly recommended to fix these problems because they threaten the stability of your build.
 [WARNING]
 [WARNING] For this reason, future Maven versions might no longer support building such malformed projects.