Remove HnswSpool

It was just used to pass heap/index relations to
HnswParallelScanAndInsert. I think it was copied from nbtsort.c, which
is more complicated. I don't think we need a struct like this.

(That said, I actually think that we should have a state object that
would hold fields like 'heap', 'index', 'procinfo', 'collation'
etc. Passing that object around would simplify the signatures of many
functions. But that's a different story).

src/hnsw.h

@@ -200,12 +200,6 @@ typedef struct HnswGraph
 	bool		flushed;
 }			HnswGraph;
 
-typedef struct HnswSpool
-{
-	Relation	heap;
-	Relation	index;
-}			HnswSpool;
-
 typedef struct HnswShared
 {
 	/* Immutable state */