refactor: cleanup unused and redundant DLB settings

What does this MR do?

Related to #1375 (closed).

This change removes unused database load balancing settings that are only targeted for phase 2, namely maxreplicalagtime and maxreplicalagbytes. I added these in advance but now that I'm working on exposing these settings in Charts I find it confusing/misleading to have them already on the codebase and documentation. We can bring them back when appropriate.

Additionally, it also removes mentions to the following settings:

• disconnecttimeout: database/sql#DB.Close already performs a graceful shutdown (closing the handler immediately to prevent new query attempts and waiting for ongoing queries to complete before returning) and we can't control how long it waits for ongoing queries to finish (there is no context input). Until we find the need for something more restrictive (if ever), I think it's better to keep it simple and let the standard lib handle it. In the worst case scenario, ongoing queries will take up to the maximum query duration (configured on PgBouncer/Patroni/PostgreSQL) to complete (15s on .com). However, given that we're only performing the close after being unable to reach out to the corresponding replica, it's far more likely that no queries are in progress when we attempt the close and therefore it should complete immediately. Regardless, I've raised an issue so that we add a metric to track this elapsed time (#1376).

• recordcheckinterval: I found this to be redundant as we already have replicacheckinterval and the latter is what we're using to determine how often we're going to refresh replicas (which in terms ends up poking the DNS record for the latest host records). So trying to keep it simple.

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