CLN: Removed copy parameter in xs_* methods
Title is self-explanatory.  Picks up where #6919 left off.

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13781?src=pr) is 85.32% (diff: 100%)

> Merging [#13781](https://codecov.io/gh/pydata/pandas/pull/13781?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **<.01%**

``` diff
@@             master     #13781   diff @@
==========================================
  Files           141        141          
  Lines         50679      50671     -8   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
- Hits          43240      43235     -5   
+ Misses         7439       7436     -3   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [474fd05...c314bc1](https://codecov.io/gh/pydata/pandas/compare/474fd0519b768efd1c7ec6555930873bf9910e00...c314bc1f71a1571cfa225a8a9b8fe2eb5ff1e208?src=pr)
 thanks @gfyoung 
