CLN: reachable cases in Reducer
compute_reduction has optional `labels` and `dummy` arguments.  In practice, either both are passed or neither is passed.  By enforcing this, we can simplify the code a bit.
Hello @jbrockmendel! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:



There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2019-11-14 15:37:22 UTC sure, can you merge master. thanks