GetDeltaMomentum() and GetDeltaEnergy() are obsolete

[mvicenzi]

The calls to GetDeltaMomentum() and GetDeltaEnergy() in FASER2TrackerSD are issuing warnings:

-------- WWWW ------- G4Exception-START -------- WWWW -------
*** G4Exception : Warning
      issued by : G4Step::GetDeltaMomentum()
This method is obsolete and will be removed soon
*** This is just a warning message. ***
-------- WWWW -------- G4Exception-END --------- WWWW -------


-------- WWWW ------- G4Exception-START -------- WWWW -------
*** G4Exception : Warning
      issued by : G4Step::GetDeltaEnergy()
This method is obsolete and will be removed soon
*** This is just a warning message. ***
-------- WWWW -------- G4Exception-END --------- WWWW -------

It's not urgent, but we should find an alternative implementation.

[benw22022]

Yeah I think that's my fault, I'll get round to fixing that when I have a moment