split old- and new-style configuration of MagFieldService into two seprate files

The file MagFieldServicesConfig.py contained both, old- and new style configuration of the AtlasFieldSvc. It imported elements of both configuration systems at module level with is very fragile. Not sure how we ended up in this situation, I suspect my semi-automated migration to CompFactory in !28944 (merged) contributed to the mess.

This MR disentangles old and new configuration by splitting it into two files.