Remove stale comment for pruning forward history.

The comment mentions the need to update |last_committed_entry_index_|,
but this is not done. It looks like this was introduced here
https://codereview.chromium.org/7078002
where the update needed to happen for the replacement case, but now this
code is not reached when replacing due to
https://codereview.chromium.org/1245433002

Bug: None
Change-Id: Ia3386cb8893702b79295213b82ec2d671a17d792
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1729798Reviewed-by: Charlie Reis <creis@chromium.org>
Commit-Queue: Kevin McNee <mcnee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#683357}

content/browser/frame_host/navigation_controller_impl.cc

@@ -2450,9 +2450,6 @@ void NavigationControllerImpl::InsertOrReplaceEntry(
 
   if (current_size > 0) {
     // Prune any entries which are in front of the current entry.
-    // last_committed_entry_index_ must be updated here since calls to
-    // NotifyPrunedEntries() below may re-enter and we must make sure
-    // last_committed_entry_index_ is not left in an invalid state.
     int num_pruned = 0;
     while (last_committed_entry_index_ < (current_size - 1)) {
       num_pruned++;