DOC: Remove deprecated use of level for agg functions

[rhshadrach]

• Ensure all linting tests pass, see here for how to run them

ref #40869, cc @phofl

[phofl]

lgtm, should we remove the examples from the userguide too?

[rhshadrach]

I assumed these would give warnings when building the docs, and there weren't any. Just did some searching through the userguide and didn't find any. However, there are documented cases of Series.sub and Series.div using level. I think these are not deprecated (but maybe should be as well?)

[phofl]

Sorry I forgot this. I changed them to using groupby when deprecating the keyword

Edit: Yes they are not deprecated

[jreback]

I assumed these would give warnings when building the docs, and there weren't any. Just did some searching through the userguide and didn't find any. However, there are documented cases of Series.sub and Series.div using level. I think these are not deprecated (but maybe should be as well?)

.sub and so on are not deprecated; I think we need the level keyword (as it provides broadcasting behavior)

[jreback]

thanks @rhshadrach