Remove allow_comments_with_issue_tracker config

Remove `allow_comments_with_issue_tracker` config.

Allowing comments should not be a issue tracker responsibility.

Having to support an extra `comments_allowed?` API in every plugin is just too much, and again, this should not be up-to the issue tracker.

First I thought to only remove the option from the issue tracker and still leave it globally, but the option name doesnt make sense `allow_comments_with_issue_tracker` , showing comments always is fine, and remove a bit of extra complexity in multi-layers of errbit.

r. @stevecrozz