Remove configuration in Pythia8_i / EnhanceSplittings UserHook
As setup now, the EnhanceSplittings UserHooks in our Pythia8_i doesn't work and causes a set fault. This is due to an uninitialized settingPtr being assessed in the constructor. Instead of having this configured in the source code, this should be done by the user in their jOs. After some discussion on this MR, since Pythia8 versions >8.304 do not need this UserHook anymore, and we will not use any Pythia8 version in master below this, this UserHook can simply be removed.