THistSvc: fix or remove buggy regHist interface

While running cppcheck on the Gaudi code base, cppcheck rightfully complained that:

GaudiSvc/src/THistSvc/THistSvc.cpp:344:32: warning: Assignment of function parameter has no effect outside the function. Did you forget dereferencing it? [uselessAssignmentPtrArg]
  if ( hist_ptr != nullptr ) { hist_ptr = hist.get(); }
                               ^

That's because the interface is missing an &:

/// Register an existing ROOT histogram TH*X with name and moved unique_ptr
/// @param [in] name      defines the histogram id/name under which it is recorded
/// @param [in] hist      transfers ownership of the histogram to the THistSvc
/// @param [out] hist_ptr for compatibility: return raw pointer to managed object to support common usage in Athena
StatusCode regHist( const std::string& name, std::unique_ptr<TH1> hist, TH1* hist_ptr ) override;

Either we have to fix this interface or remove it entirely (I am checking how widely this is used in ATLAS).