Remove setNextScore in SearchScript.

While it would be nice to do this all the way up the chain (into
score functions), this at least removes the weird dual
setNextScore/setScorer for SearchScripts.
What do you mean by the "map" lookup? SearchLookup handles maintaining the scorer (since it is `ScorerAware`).  I'm sure this can be simplified in the future, but this PR was just to start by getting rid of `setNextScore`.