Issue #3611367: Do not cancel the whole setup when clicking Skip and finish

The "Skip" / "Skip and finish" button in TfaSetupForm::buildForm() carried '#submit' => ['::cancelForm'], so it ran cancelForm() ("Setup of ... canceled.") and aborted the whole setup instead of skipping the current step. The dedicated skip branch in submitForm() (isset($values['skip']) && $values['op'] === $values['skip']) was dead code.

Change: remove the '#submit' override (keep '#limit_validation_errors' => []) so the button falls through to submitForm() and hits the skip branch. With limited validation the triggering value is still written to both $values['op'] and $values['skip'] by FormValidator::handleErrorsWithLimitedValidation(), so the comparison holds. The first/single-step "Cancel" button keeps ::cancelForm.

One hunk in src/Form/TfaSetupForm.php.

Precondition to reproduce: #3451488 / MR !163 (the multi-step path is only reachable once the setup-plugin re-resolve fix is applied).