CLN: Remove leftover partial-result code from apply
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

hmm yeah i don't really recall while this was needed

any perf differences? @jreback I added the partial results in order to solve https://github.com/pandas-dev/pandas/issues/33879 ( PR https://github.com/pandas-dev/pandas/pull/34183)
But it seems that the whole use of `libreduction` was removed, and there is no need.

All the code i removed isn't actually used, so there shouldn't be any perf difference

Do you happen to know why `libreduction` isn't used anymore? is there a slowdown after its removal? libreduction just added complexity 

 ok.
Anything left for this to be merged? yeah this is nice @alonme thanks