runtime: remove redundant "//"

Change-Id: I44af6cd8baa8fcd2a666429467fae4bed5f21fa0
GitHub-Last-Rev: d0c9722f082114b356de55fc79ce138ede019636
GitHub-Pull-Request: golang/go#32052
Reviewed-on: https://go-review.googlesource.com/c/go/+/177277Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>

src/runtime/runtime2.go

@@ -736,7 +736,7 @@ type itab struct {
 }
 
 // Lock-free stack node.
-// // Also known to export_test.go.
+// Also known to export_test.go.
 type lfnode struct {
 	next    uint64
 	pushcnt uintptr