Remove the parameter goalPrefix (and corresponding code) from the archetype:add-archetype-metadata mojo

As questioned in Archtypes and plugin prefix, it appears the parameter goalName of the add-archetype-metadata mojo exists merely due to copy&paste but has no real use in the context of archetypes. As such the parameter and its backing code should be removed to reduce confusion for end users and to keep the group-level metadata free from bogus plugin prefix mappings for archetypes.