Resolution for issue #652 - Removed unused index by_feed_expiration

[jmjatlanta]

The index 'by_feed_expiration' is unused. This change removes the index by removing the code.

[oxarbitrage]

looks good John, compiles and synchronizing fine as well.
interesting to see that the struct by_id is not needed. maybe we can remove it from other places as well. what do you guys think ?

[abitmore]

We want to remove the by_feed_expiration index because it will consume a little RAM and computing resources in production (need to insert/update when data changes). The by_id tag doesn't do any harm.