Remove comment_add and inline final occurrence

It should just be removed.
There's an issue to allow render cache callbacks to be services which converts the static call to a service. That leaves only one call to comment add in the form. It can be inlined.

BTW it is decoupled from node

Actually that issue is fixed #2247779: Allow #pre_render, #post_render and #post_render_cache callbacks to be service methods so comment add can go inline for the remaining call(s) (IMO). My motivation in https://www.drupal.org/node/2247779 was to kill comment add

+++ b/core/modules/comment/src/Plugin/Field/FieldFormatter/CommentDefaultFormatter.php
@@ -174,7 +187,17 @@ public function viewElements(FieldItemListInterface $items) {
+            $field = Fieldconfig::loadByName($entity_type, $field_name);

We should have the field in scope here, no need to use the static. $this->getFieldDefinition() from memory

A couple of fixes:
1) there's comment_type not a "bundle" - take this directly from field
2) use entity form builder variable to prevent collisions with form builder

Suppose this one rtbc

Committed 4b91f16 and pushed to 8.x. Thanks!

The original commit included #2292197: The StringTranslationTrait trait doxygen could use a little love - sorry.

Committed e7f8c6d and pushed to 8.x. Thanks!