search: structural search doesn't depend on global Zoekt eval

Created by: rvantonder

Context: I'm trying to simplify yucky stuff around search job creation. This is setup for a more involved simplification.

This PR: Before separating "search over indexed repositories" for different text search vs structural search, we controlled whether that path would be executed for global searches. Back then, structural search always executed along the "search over indexed repositories" path, so over the course of mechanical factoring, it kept this decision to "search over indexed repositories only when we didn't run global mode". It is, and should be, agnostic to whether global mode runs or not, because it calls Zoekt from searcher with a resolved set of repos. We can and should optimize the global case for it, but historically it never was, and it shouldn't depend on this value now either way.

Test plan

Semantics-preserving.