runtime: resolve latent TODOs

These were added in https://go-review.googlesource.com/1224; according
to austin@google.com these annotations are not valuable - resolving by
removing the TODOs.

Change-Id: Icf3f21bc385cac9673ba29f0154680e970cf91f2
Reviewed-on: https://go-review.googlesource.com/c/go/+/176899Reviewed-by: Austin Clements <austin@google.com>

src/runtime/mgc.go

@@ -1991,7 +1991,6 @@ func gcMarkWorkAvailable(p *p) bool {
 // gcMark runs the mark (or, for concurrent GC, mark termination)
 // All gcWork caches must be empty.
 // STW is in effect at this point.
-//TODO go:nowritebarrier
 func gcMark(start_time int64) {
 	if debug.allocfreetrace > 0 {
 		tracegc()

src/runtime/mgcmark.go

@@ -255,8 +255,6 @@ func markrootBlock(b0, n0 uintptr, ptrmask0 *uint8, gcw *gcWork, shard int) {
 //
 // This does not free stacks of dead Gs cached on Ps, but having a few
 // cached stacks around isn't a problem.
-//
-//TODO go:nowritebarrier
 func markrootFreeGStacks() {
 	// Take list of dead Gs with stacks.
 	lock(&sched.gFree.lock)

src/runtime/mgcsweep.go

@@ -203,7 +203,6 @@ func (s *mspan) ensureSwept() {
 // Returns true if the span was returned to heap.
 // If preserve=true, don't return it to heap nor relink in mcentral lists;
 // caller takes care of it.
-//TODO go:nowritebarrier
 func (s *mspan) sweep(preserve bool) bool {
 	// It's critical that we enter this function with preemption disabled,
 	// GC must not start while we are in the middle of this function.