TST/CLN: roll_sum/mean/var/skew/kurt: simplification for non-monotonic indices
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

The removed for-loop doesn't seem to be necessary (I hope this code is tested by an existing test).

I feel like I'm missing an obvious reason why these for-loops are needed: looking at the code I don't think we need them and the tests also pass.