Cleanup unused tool in Det/Magnet and move lookup table class to LHCbKernel

Based on comments in Rec!1554 (merged), PrTableForFunction was renamed to LUTForFunction and moved to LHCbKernel. BdlTool in Det/Magnet is not referred to in recent versions of neither Brunel nor DaVinci; it looks like PrVeloUTMagnetTool in Pr/PrVeloUT replaced BdlTool with a more UT-specific implmentation. I choose to remove BdlTool and put LUTForFunction in Kernel/LHCbKernel, because Det/Magnet has no public headers.

I've added the same slot as Rec!1554 (merged) to make sure that MR keeps building