Remove `shippingLabelsOnboardingM1` feature flag

[pmusolino]

Remove the "Get WooCommerce Shipping" banner, by removing shippingLabelsOnboardingM1 feature flag, as mentioned here.

[dangermattic]

🚫 Please add a type label (e.g. type: enhancement) and a feature label (e.g. feature: stats) to this issue.