[MPLUGIN-480] Do no longer generate plugin-help.xml for help mojo

**[Konrad Windszus](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=kwin)** opened **[MPLUGIN-480](https://issues.apache.org/jira/browse/MPLUGIN-480?redirect=false)** and commented

Currently the ["helpmojo" goal](https://maven.apache.org/plugin-tools/maven-plugin-plugin/helpmojo-mojo.html) generates a dedicated plugin descriptor named "plugin-help.xml" which ends up in the plugin package next to the regular descriptor. As the former is only a restricted version of the latter, there shouldn't be no need to have a dedicated "plugin-help.xml" in the plugin jar and instead rather rely only on the official descriptor also for the help mojo. Compare with https://maven.apache.org/plugin-tools/index.html#plugin-descriptors

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**Affects:** 3.9.0

The issue identifies the helpmojo goal and plugin descriptors as the entry points; start by tracing how helpmojo produces plugin-help.xml and inspect the related package/output tests. Done means the help mojo no longer adds plugin-help.xml to the plugin JAR while the official descriptor remains available.

Written by the indexing model from the issue text.