Delete PhiHelper

The methods have been moved to `CxxUtils/phihelper.h`. Note that
the equivalent of `HLT::phiMean` is `CxxUtils::phiBisect`.

Trigger/TrigEvent/TrigSteeringEvent/TrigSteeringEvent/PhiHelper.h

-// emacs: this is -*- c++ -*-
-/*
-  Copyright (C) 2002-2018 CERN for the benefit of the ATLAS collaboration
-*/
-//
-//   @file    phiHelper.h        
-//
-//
-//   $Id: phiHelper.h, v0.0   Sun 20 Jun 2010 20:21:49 BST sutt $
-
-
-#ifndef PHIHELPER_H
-#define PHIHELPER_H
-
-namespace HLT {
-
-inline double wrapPhi( double phi ) { 
-    static const double M_2PI = 2*M_PI;  
-    while (phi> M_PI) phi -= M_2PI;
-    while (phi<-M_PI) phi += M_2PI;
-    return phi;
-}
-
-
-inline double phiMean( double phimin, double phimax) { 
-    double phi = 0.5*(phimin+phimax);
-    if ( phimin>phimax ) phi += M_PI; 
-    return wrapPhi(phi);
-}
-
-
-inline double deltaPhi( double phimax, double phimin) { 
-  return wrapPhi(phimax-phimin);
-}
-
-}
-
-
-#endif  // __PHIHELPER_H