CLN: implement xs in terms of loc
after #6134 we can drop xs and implement directly. Thus we should deprecate it.

`.xs(key, level=n)` 

is roughly equivalent to this

```
indexer = tuple([slice(None)]*(n-1) + key)]
axis_indexer = [ slice(None) ] * self.ndim
axis_indexer[axis] = indexer
self.loc[tuple(axis_indexer)]
```

roughly because needs to handle `.xs` full argument set
- drop_level (need to surround the indexer tuple with a list or not)
- n could be a level name 
- n = 1 (needs a test)
