MuonGeomTest: Fix placement of using directive.

Don't put a using directive in the global namespace in headers.

MuonSpectrometer/MuonValidation/MuonGeomTest/src/MuonGMCheckCorners.h

@@ -14,7 +14,6 @@
 #include "GaudiKernel/NTuple.h"
 #include "MuonReadoutGeometry/MuonReadoutElement.h"
 
-using namespace MuonGM;
 
 class MuonGMCheckCorners : public AthAlgorithm {
 public:

MuonSpectrometer/MuonValidation/MuonGeomTest/src/components/MuonGMCheckCorners_entries.cxx

@@ -2,4 +2,4 @@
 #include "../MuonGMNtupleWriter.h"
 
 DECLARE_COMPONENT(MuonGMCheckCorners)
-DECLARE_COMPONENT(MuonGMNtupleWriter)
+DECLARE_COMPONENT(MuonGM::MuonGMNtupleWriter)