Fixed minor misses in Muon rename
When integrating !218 (merged) in !204 (merged) I found a couple of issues:
- Detector/Muon/src/old/muon_geo.cpp was not correct (undetectable as it is never used)
- Detector/Muon/include/Detector/Muon/Namespace.h not updated (used only in LHCb)
This MR addresses those issues