Remove vestigial isDemoDomain branch in email-domains.ts (post VENUS-270)

Once VENUS-270 removes password-login-actions.ts + the @demo.venus password-login gate, the isDemoDomain allowlist branch in email-domains.ts (VENUS-269 file) has no live caller — no real Google OAuth account can ever present a @demo.venus email, so the branch only ever existed to let password-login's fake accounts pass the domain gate. Small, low-risk cleanup, deliberately kept OUT of VENUS-270 to avoid re-touching a just-shipped Class-A file in the same diff. Flagged by coder-venus-cc during VENUS-270 scoping.