Remove `prep_merge` callback
Remove `prep_merge` callback

Summary: I'm not sure what the history here is, but it doesn't seem to serve any useful purpose anymore. We might as well just do our logging before calling `merge`.

Reviewed By: gabelevi

Differential Revision: D18310861

fbshipit-source-id: de2606413734f00ae63759c9696b8d36ce460533

facebook/flow,samwgoldman/flow,facebook/flow,facebook/flow,facebook/flow,facebook/flow,mroch/flow,mroch/flow,facebook/flow,samwgoldman/flow,mroch/flow,mroch/flow,mroch/flow,mroch/flow,samwgoldman/flow,samwgoldman/flow,facebook/flow,samwgoldman/flow,mroch/flow,samwgoldman/flow,samwgoldman/flow