"BrainStemMask" not in julich brain 3.0 parcellation but in the fsaverage map

"BrainStemMask" is not a part of the jba3 parcellation but in fsaverage map. Therefore, if one tries to get_region("BrainStemMask") from the map, it will fail.

But to create the fsaverage map, Harmut needs to create a "BrainStemMask".

Suggested solution: removed "BrainStemMask" from fsaverage jba3 map as it is not part of the parcellation and it only serves a need for creating the original surf map.