WebMapService class should be reduced to what actually differentiates it from the super class.

[JanEisermann]

I quote @matrss to describe this issue.

"[The WebMapService] class seems to copy quite a bit from its super class, both in the constructor and in _buildMetadata and ContentMetadata. _buildMetadata has exactly the same issue as the constructor, since it does not call the super class implementation even though it does something very similar. I think this subclass should be reduced to what actually differentiates it from the super class."

This issue originated from the following issue: #2750