CLN: remove never-used branch from Series ops
_Almost_ gets to a point where Series and DataFrame ops can share the na_op definition.

AFAICT this is the only non-test usage of libalgos.arrmap_object.
So what's the conclusion re arrmap_*? pandas/core/algorithms.py
```
        q = np.asarray(q, np.float64)
        return algos.arrmap_float64(q, _get_score)
```

I think you can just do ``np.vectorize`` on this and it will work, if so, then let's completely blow away arrmap I'll open an issue to look into this more carefully. cool, ping on green. ping thanks @jbrockmendel 