Revert "Regression in merge_asof raising KeyError when at least one by column is in index
I am really sorry for this.

I messed up my acces token and accidentially pushed to master. Not sure how this could happen :(

If someone has a better idea than simply reverting we can close this

cc @pandas-dev/pandas-core 

I did something similar about 6 months ago. No worries. We reverted that. Thanks for the quick help, merging this then

Comforting, that I am not the only one this happened to  yep no problemo!