transform: remove dead code

[petrosagg]

@benesch @wangandi I put you as reviewers based on git blame. These files are not included in any cargo target as far as I can tell

[benesch]

Replacing myself with @frankmcsherry because I don't know what's desired here!

[frankmcsherry]

I suspect these are probably fine to delete. They represent work that was eventually done a different way, but without the conviction that we should remove the alternative approaches. I think aggregation.rs was mine (and was about reshaping Reduce in a way that we have issues filed for). I think use_indexes.rs was @wangandi and was about using indexes for indexed access to data (which we have in MZ now).