Remove the MuonStraightLineExtrapolator from the MuonTrackCleaner
As far as I can tell there is no difference between the MuonStraightLineExtrapolator and the ordinary Extrapolator, so there's no reason for the MuonTrackCleaner to have both of them. RunTier0Tests reports no differences from removing the former and using the latter instead.
Closes ATLASRECTS-6328
Let's ping @amete since this should also have an effect on number of extrapolator instances.
Edited by Christos Anastopoulos