internal/lsp/cache: remove unused function

This function was actually left behind, after making suggested changes
to CL 413683.

Change-Id: I6933e870ded9da5af06724c28839c37d58fb4cdc
Reviewed-on: https://go-review.googlesource.com/c/tools/+/414856
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>

internal/lsp/cache/graph.go

@@ -158,18 +158,3 @@ func (g *metadataGraph) reverseTransitiveClosure(includeInvalid bool, ids ...Pac
 	visitAll(ids)
 	return seen
 }
-
-func collectReverseTransitiveClosure(g *metadataGraph, includeInvalid bool, ids []PackageID, seen map[PackageID]struct{}) {
-	for _, id := range ids {
-		if _, ok := seen[id]; ok {
-			continue
-		}
-		m := g.metadata[id]
-		// Only use invalid metadata if we support it.
-		if m == nil || !(m.Valid || includeInvalid) {
-			continue
-		}
-		seen[id] = struct{}{}
-		collectReverseTransitiveClosure(g, includeInvalid, g.importedBy[id], seen)
-	}
-}