compiler warning from BDSCollimatorRectangular

[Jochem Snuverink]

bdsim/src/BDSCollimatorRectangular.cc:42:16: warning: 
      unused variable 'newLength' [-Wunused-variable]
      G4double newLength = chordLength*0.5 + 2*lengthSafetyLarge;

[William Shields]

Line removed from the file.