CLN: small clean-up of IntervalIndex
Removed some parts of IntervalIndex that are not needed (because they are inherited from Index or can be dispatched to the array) 
\+ removed `fastpath` (for IntervalArray this is no problem (never used), for IntervalIndex I am not fully sure, since this signature might need to match the other index classes)

cc @jschendel @TomAugspurger 