[Singleton Service APIs] Remove fallback to en content when the translation no exist for request locale

[nannany00]

Singleton Service translation-related APIs. it has a function to fallback the translation to EN content when no translaiton exist for request locale. But it's not flexible when the source locale is not English for product, so we remove this in Singleton Service, and let Singleton Client do which content will be returned when translaiton is not ready.

[nannany00]

Based on our current user usage, we only do this change for v2 API that Singleton Client used:
GET /i18n/api/v2/translation/products/{productName}/versions/{version}/locales/{locale}/components/{component}