UNOMI-915: remove useless usage on maven-source.plugin.version

[jsinovassin]

The parent of unomi

<parent>
       <groupId>org.apache</groupId>
       <artifactId>apache</artifactId>
       <version>29</version>
       <relativePath />
   </parent>

already defines the maven-source-plugin in version 3.2.1

As we upgraded the version of this plugin, its not necessary to add the definition the plugin in the Unomi parent.
If we keep it it will generate a duplication which is not supported anymore with the version 3.2.1 of maven-source-plugin