Remove snapshot window logic, see SERVER-31767.

• This logic was used to dynamically change the amount of history stored to avoid cache pressure. Given durable history is available now, this is no longer needed.
• Delete metrics in the "snapshot-window-settings" subsection of the wiredTiger serverStatus section that are no longer relevant.
  • "cache pressure percentage threshold"
  • "max target available snapshots window size in seconds”
  • "target available snapshots window size in seconds"

Add metric "minimum available snapshots window size in seconds".