chore: remove no longer reachable code path (#5503)
This path is no longer reachable because the query graph for updates now
always connect selectors for updates. This worked on MSSQL because it
supports selectors even though it doesn't have the `UpdateReturning`
capability. If we want MSSQL to leverage this we should look into
enabling the capability instead. It'll make MSSQL query graphs simpler
and likely faster to execute.
