Remove deprecated comment_num_new()

Other issue is in so unpostponing,.

Please review.

Confirming that the patch in #3 applies cleanly for me, and that the "new comment count" continues to function as expected when commenting on articles.

To test this I did the following

1. Read the patch to confirm that the function is indeed deleted
2. Applied the patch, and confirmed that drupal continued booting
3. Created an article, added comments, and viewed the article teaser as a different user to confirm that the comment count continues functioning
4. Ran Drupal\comment\Tests\CommentLinksTest in the browser to confirm that all tests still pass.

However, one small item I noticed that could use a bit of additional work: in core/modules/comment/src/Tests/CommentLinksTest.php on lines 234 and 246 there are docblocks which reference comment_num_new(). The overall test remains relevant and functional, but this documentation should be updated to reflect the new function name. I'll update and post a new patch in a moment.

This patch 1) removes the deprecated function comment_num_new() and 2) replaces all docblock references to comment_num_new() with the new method \Drupal::service('comment.manager')->getCountNewComments()

Here's an interdiff for the above patch

Yup, that docblock is indeed still accurate.

Here's the behavior that docblock is describing:

1. User Alice lacks access to comments.
2. User Bob makes a comment on node/123
3. User Alice views node/123. User Alice does not see user Bob's comment. User Alice closes the tab.
4. User Alice receives a new user role, and is now able to access comments.
5. User Alice views the teaser for node/123
6. User Alice should not see "1 new comment," because Bob's comment was marked as read the last time she visited node/123 (despite the fact that she never saw Bob's comment in reality).

I've tested this behavior by creating dummy user accounts and dummy content, and it is indeed preserved in getCountNewComments()

Committed 46a9a49 and pushed to 8.x. Thanks!