[ACTV-273] Remove feature flag for the onboarding and step deck tours

[augdiebold]

Related issues

• Closes #

Proposed changes

Graduate the onboarding_tour, step_deck_tour, and tutorial_metrics_tracker add-on feature flags to always-on. The tours, Help menu entries, first-sync intro-deck subscribe, and tutorial ProductMetrics tracking stay; only the remote flag checks are removed.

Local conditions are unchanged:

• Onboarding still requires a logged-in user and first-profile timing (last_deck_sync is None).
• The AnKing Step Deck tour still requires the AnKing deck to be installed.
• Intro-deck auto-subscribe still runs only on first sync when that deck is not already configured.

Flag-off tests and the get_feature_flags mocks that existed only to enable these flags are removed.

How to reproduce

1. Log in on a profile that has never synced (last_deck_sync is empty). Confirm the onboarding prompt still appears (after login and/or on first AnkiHub sync).
2. On Help → Product tours, confirm Onboarding is listed while logged in, with no dependency on a remote flag.
3. On first AnkiHub sync of a new profile that does not already have the intro deck, confirm the client still subscribes to the intro deck.
4. Install the AnKing Step Deck. Confirm the step-deck tour prompt can appear, Help → Product tours includes AnKing Step Deck, and the overview tour link shows when that deck is selected.
5. Start a tour and step through / skip / finish it. Confirm tutorial events still go to ProductMetrics (no tutorial_metrics_tracker gate).