controllers: forward pullrequests.delete_comment to changeset

Remove duplication between pullrequests and changeset.
We move the code outside ChangesetController to make it callable from
PullrequestsController.

Note:
- instead of keeping the method pullrequests.delete_comment itself and
letting it forward to changeset.delete_comment, an alternative solution
would have been to change the routing table directly. However, the chosen
solution makes it more explicit which operations are supported on each
controller.