Remove plugin parent_select unused symbols

[rob05c]

These exclude_hostname and exclude_port args were needed due to limitations on events, the parent_select plugin needed to get the subsequent parent to try, if necessary, because no event would fire. So it needed to say, "give me the next parent if this one were marked down, even though it isn't."

Their usage got lost in the parent_select consistent hash refactor (#8590), which I thought would break failover.

But I just extensively tested failover, for DNS, network, and internal code failures, and everything worked up to 3 tries. So it seems either #8590 (#7925 and #8365) or possibly the recent HostDB refactor (#8953) changed something to not make it necessary.

So this PR cleans up the parameters and variables, and just removes them entirely.

This is purely cleanup, doesn't affect behavior. But I recommend backporting to 9.2.x just to reduce future merge conflicts.