Sparse Ops Cleanup
This is in preparation for implementing the direct-pinning discussed [here](https://github.com/pandas-dev/pandas/pull/19649#discussion_r167527010).  The main roadblock to that is SparseSeries which calls ops.add_special_arithmetic_methods twice.  This PR gets rid of the double-call, so the follow-up will be straight-forward.

A handful of cleanups are done here too:
 - unrelated docstring fix for TimedeltaIndex.replace
 - minor formatting improvements
 - remove an unused kwarg for DataFrame._compare_frame
 - remove a loop in add_flex_arithmetic_methods, replace with an assertion that it is unnecessary
 - fix names for some SparseArray method names, e.g. ATM `SparseArray.__eq__.__name__ == 'eq'`
