REF: remove _convert_scalar_indexer
This sits on top of #31867, so is partially a demonstration of how much complication is caused by our inconsistent error-raising.
@jbrockmendel can you merge master xref #25996 Definitely simplifies things so +1 there. Does this need to get bundled with the other PR to comprehensively raise a `KeyError`? Again on board, just want to be careful of messaging and backwards compat for users that previously caught TypeError (if even an issue - maybe they always caught both?) > Does this need to get bundled with the other PR to comprehensively raise a KeyError?

Yes. closing to clear the queue, will re-open following #31867. re-opened following #31867 love it. if you can rebase (not sure if it was before). merge on green. Thanks @jbrockmendel 