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 
Hello @jorisvandenbossche! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/core/arrays/interval.py`](https://github.com/jorisvandenbossche/pandas/blob/d9860f453b8b69f6f5fa63606f937dcab4f07630/pandas/core/arrays/interval.py) !

 - There are no PEP8 issues in the file [`pandas/core/indexes/interval.py`](https://github.com/jorisvandenbossche/pandas/blob/d9860f453b8b69f6f5fa63606f937dcab4f07630/pandas/core/indexes/interval.py) !

