Remove redundant subscriptions.init() from EA API

[teiswit]

It is not needed to call subscriptions.init(productId) when a productId is provided in the page markup. Furthermore this init call can result in an error if the page has already been initialized outside of EA. This is causing an issue with 3P integrations where SwG.js may have been initialized by method calls like getEntitlements() prior to GaaMetering.init.

Therefore I propose the folllowings:

1. Remove the redundant subscriptions.init() from EA API
2. Re-purpose GaaMetering.getProductIDFromPageConfig_() for a validation purpose i.e. ensuring the page has productId in the markup

Internal tracking: b/271063736