chore: remove no longer reachable code path

[jacek-prisma]

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.

[codspeed-hq]

CodSpeed Performance Report

Merging #5503 will not alter performance

_{Comparing chore/remove-unreachable-path (8f97485) with main (6cbdedf)}

Summary

✅ 11 untouched benchmarks

[github-actions]

WASM Query Engine file Size

Engine	This PR	Base branch	Diff
Postgres	2.184MiB	2.184MiB	0.000B
Postgres (gzip)	874.495KiB	874.493KiB	2.000B
Mysql	2.149MiB	2.149MiB	0.000B
Mysql (gzip)	859.676KiB	859.676KiB	0.000B
Sqlite	2.059MiB	2.059MiB	0.000B
Sqlite (gzip)	825.415KiB	825.415KiB	0.000B
SQL Server	2.115MiB	2.115MiB	0.000B
SQL Server (gzip)	849.280KiB	849.279KiB	1.000B