[IAP] When retrieving plan availability, do so from the API only

[iamgabrielma]

From #9911 (comment)

extension UpgradesViewModel {
    enum AvailableInAppPurchasesWPComPlans: String {
        case essentialMonthly = "debug.woocommerce.express.essential.monthly"
    }
}

Update 30.07: Noting that we use this enum as type when discerning between different styles within the UI, once we remove the code from M1 it should be easier to remove as needed.

public enum AvailableInAppPurchasesWPComPlans: String {
    case essentialMonthly = "woocommerce.express.essential.monthly"
    case essentialYearly = "woocommerce.express.essential.yearly"
    case performanceMonthly = "woocommerce.express.performance.monthly"
    case performanceYearly = "woocommerce.express.performance.yearly"
}