sa: investigate removing requestedNames table

[jsha]

trafficstars

requestedNames is our 3rd biggest table by row count and by byte size, and I suspect we don't need it. It's used in GetOrder to calculate order.Names but we could calculate the same thing using the fetched authzs. Investigate the history here and see if we can get rid of it.

[jsha]

The table was added in https://github.com/letsencrypt/boulder/pull/3169. There wasn't significant discussion about whether it would be better to get the relevant info from the authorizations table. Reading that PR and the code I'm convinced that we can remove the requestedNames table easily. I've filed #7435 as step 1. Once that's deployed, we won't be querying the table anymore and we can file a followup PR that removes writes to the table and drops it from the DB.