runtime: remove unused parameter from bestFitTreap

This code was added recently, and it doesn't seem like the parameter
will be useful in the near future.

Change-Id: I5d64dadb6820c159b588262ab90df2461b5fdf04
Reviewed-on: https://go-review.googlesource.com/39692
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/mheap.go

@@ -750,13 +750,13 @@ func (h *mheap) isLargeSpan(npages uintptr) bool {
 
 // Allocate a span of exactly npage pages from the treap of large spans.
 func (h *mheap) allocLarge(npage uintptr) *mspan {
-	return bestFitTreap(&h.freelarge, npage, nil)
+	return bestFitTreap(&h.freelarge, npage)
 }
 
 // Search treap for smallest span with >= npage pages.
 // If there are multiple smallest spans, select the one
 // with the earliest starting address.
-func bestFitTreap(treap *mTreap, npage uintptr, best *mspan) *mspan {
+func bestFitTreap(treap *mTreap, npage uintptr) *mspan {
 	return treap.remove(npage)
 }