Fix « mark unread » issue with Inoreader (issue #390)

Issue #390 is caused by an error on HTTP request.

According to API documentations (https://github.com/bazqux/bazqux-api, http://feedhq.readthedocs.org/en/latest/api/reference.html, http://wiki.inoreader.com/doku.php?id=api and https://github.com/theoldreader/api) :
- the `kept-unread` tag is only supported by FeedHQ.org
- the `tracking-kept-unread` tag is not supported by any of our supported services

Removing this code is OK with FeedHQ.org, so we don’t need to mind.