Use MagneticFieldGrid from Detector instead of DetDesc
With !3107 (merged) we got a copy of MagneticFieldGrid
in Detector and now that we can use code from Detector even in DetDesc mode, we should always use the copy from there.
This MR removes the copy in LHCb to use only that in Detector.
Note: It requires Rec!2863 (closed) Rec!2860 (merged)
Edited by Marco Clemencic