update elastic-recheck docs about bug query removal process

There was a misunderstanding of how we remove old bug queries from elastic-recheck here:

https://review.openstack.org/#/c/204133/

We should codify that process in the readme so there isn't confusion.

Replaying my comment here:

1. You shouldn't remove queries that have "suppress-graph: true" in them since we basically want to keep those around, they are persistent infra issues and aren't going away.

2. You're removing queries for things that still have hits. I think because the bugs are marked as fixed or invalid, but if they are still hitting we have to question if they are actually fixed or invalid. In some cases, like infra 'wont fix' issues, they might be candidates for adding 'suppress-graph: true' - I commented on one like that.

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The generally process for this kind of cleanup is really just:

1. go to this page: http://status.openstack.org/elastic-recheck/index.html

2. look for anything that is grayed out at the bottom which means it hasn't had any hits in 10 days

3. from those, look for the ones that are fixed/incomplete/invalid/wont fix - those are candidates for removal.