Remove unused index backoff logic

[jtibshirani]

This PR removes the 'backoff' logic for repos that fail to index. To the best of my knowledge, this is not used anywhere. And we should now be less susceptible to "poison" repos that constantly fail to index:

• In the index queue, we make sure to deprioritize repos that previously failed to index
• We are focused on fixing the root cause of index failures, and don't have too many repos failing on dot com anymore