Enhancement: feed_links_extra needs filter to remove a feed from pages

[sgatz]

Use case: If your site doesn't have comments, or uses a service like disqus, the singletitle feed is useless and always empty (since comments are open, just not using wordpress native commenting). I'd like to be able to remove it from the header. Other use cases are if people do not want to provide an RSS feed for a tags or authors.

Suggestion: allow a filter on feed_links_extra to eliminate the echoing of that feed into the header. This ticket proposes filtering the default values: https://core.trac.wordpress.org/ticket/43225 Perhaps modify that to accept null or false values for things like 'singletitle' and if false, skip the creation of that item.