Search backend: simplify newToMatches

Created by: camdencheek

This is just a small style change. At some point I'd implemented this as a function that returns a closure over its arguments, but I think that was before we had streaming for searcher so we weren't already creating a closure here. In any case, when pairing with @tbliu98, he rightly asked why we did it in this weird and convoluted way and I didn't have a real answer, so I'm making it less confusing by just calling the function with its full set of arguments every time.

Stacked on #36432

Test plan

Should be semantics-preserving, but I'm depending on a quick manual smoke test and existing tests.