refactor(verify): drop the unreachable unknown optimizer runs branch

[mkzung]

Follow-up to #16055, which @mattsse closed because the premise was wrong: the CLI path
normalizes optimizer_runs, so nothing user-facing was broken. This is the other option
offered there: the branch removed, and called dead code rather than a bug.

find_mismatch_in_settings runs only from print_result, whose four callers all sit in
bytecode.rs and pass the Config that comes from load_config. That normalizes
optimizer_runs, so it is always set by then and the None arm cannot print.

No behaviour change, so no test. The unit tests in the closed PR failed only because they
built Config by hand and skipped config loading, which is what made the premise look true.

Claude Code was used for implementation assistance, covering the code change and this
description. All changes have been reviewed, understood and manually tested by me.

[github-actions]

✅ Changelog found

The deterministic check will validate the changed entry.

[mablr]

sgtm

[mkzung]

Rebased onto master and added a .changelog entry, so the two red checks are gone. The changelog gate landed after this PR opened, and cargo deny was flagging RUSTSEC-2026-0258 because the branch still pinned h2 0.4.15 while master is on 0.4.16.

Sorry about the dismissed approvals, the rebase took them with it. The diff is unchanged apart from the new fragment.