Update documentation on how to remove obsolete shim's

I built the following PS snippet to clean my choco bin path (in an admin-shell) from obsolete shim's:

```ps1
gci C:\ProgramData\chocolatey\bin\*.exe | %{ if( $( & $_.FullName --shimgen-help | Select-String "Target exists: 'False'" -Quiet ) ) { rm $_.FullName } }
```

I think its helpful, so maybe you want to include it in your documentation. Not sure if this should be included here or [there](https://github.com/chocolatey/choco/wiki/FeaturesShim).

┆Issue is synchronized with this [Gitlab issue](https://gitlab.com/chocolatey/collaborators/shimgen/-/issues/15) by [Unito](https://www.unito.io)