Review maven-clean-plugin usage

[xsalefter]

We define maven-clean-plugin here. But AFAIK, we not use it anywhere. My understanding is that, to "use" that plugin, we should define it in build/plugins/plugin. So far, we just declare/define it in build/pluginManagement/plugins/plugin. I checked any other project that may use it, but I can't found any. The only project that use it is api-pojos, but this project not using this parent pom.

[pierre]

IIRC some of the "built-in"/"special" plugins (clean, compile, etc.) don't need to be explicitly defined in the plugins section. We only define it in the pluginManagement to force the right version.