ITk fast tracking SP seed maker merging

This MR aims at:

• cleaning some unused members in the ITk default SP seed maker, such as the ones related to PPS seeding
• merging the ITk fast tracking SP seed maker into the default one, the fast tracking implementation being switched on thanks to the proper configuration

Overall, the goal is to make the maintenance of the module easier and to better highlight the differences between default and fast tracking in view of future developments.

I'm checking that atm identical behaviour is preserved before and after the MR but I've already identified some places where some unintended change was introduced in the porting of fast tracking from 20.20. I will restore the original implementation. I will also have another pass to try to check whether all of the if(m_fastTracking) statements are really necessary or if things can be further harmonised.

Edit: checks done, ready to be merged

FYI @ncalace @nstyles @gavrilen @npetters @goblirsc