CLN: Drop lags parameter from Panel.shift
Title is self-explanatory.

xref #6928

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/14041?src=pr) is 85.25% (diff: 100%)

> Merging [#14041](https://codecov.io/gh/pydata/pandas/pull/14041?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **<.01%**

``` diff
@@             master     #14041   diff @@
==========================================
  Files           139        139          
  Lines         50381      50380     -1   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
  Hits          42953      42953          
+ Misses         7428       7427     -1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [a01e58f...8cf5557](https://codecov.io/gh/pydata/pandas/compare/a01e58fa780ed28f983e1a022b07e196e037d42a...8cf5557e83fe9c0b828ccd37b42a9a63bb8898f7?src=pr)
 @gfyoung Thanks!
