runtime: remove unused unique.runtime_blockUntilEmptyFinalizerQueue

It was added in CL 650256, and then the use in the unique package
was removed in CL 650697.

Change-Id: Id95f5dff7e11a2dc3eb544fda2586a305d3d91ab
Reviewed-on: https://go-review.googlesource.com/c/go/+/681476
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

Author: cherrymui

src/runtime/mfinal.go

@@ -338,11 +338,6 @@ func blockUntilEmptyFinalizerQueue(timeout int64) bool {
 	return false
 }
 
-//go:linkname unique_runtime_blockUntilEmptyFinalizerQueue unique.runtime_blockUntilEmptyFinalizerQueue
-func unique_runtime_blockUntilEmptyFinalizerQueue(timeout int64) bool {
-	return blockUntilEmptyFinalizerQueue(timeout)
-}
-
 // SetFinalizer sets the finalizer associated with obj to the provided
 // finalizer function. When the garbage collector finds an unreachable block
 // with an associated finalizer, it clears the association and runs