Avoid automatic subscriptions for content types where notifications are not enabled - or at least delete them by content type

Issue Summary

Hi,

in my project many content types are not enabled for notifications. As usually users have automatic subscription for there nodes and comments enabled many useless subscriptions are automatically created and saved in database because even for content type where subscriptions are not enabled. Is there a way to avoid these automatic subscriptions for these content types or at least delete all subscriptions by content type? now in admin section the subscriptions cannot be filtered by content type.

best,
Tobias