TIF: Remove countryCode in TvContentRating

In the standard TV content rating systems, there are no cases that
a TV content rating system is used for multiple countries.
Moreover, since we allow the case that the country code is null for
a custom rating system (e.g. YT), the country code should be removed.

Change-Id: I79472dbd491aa7efb40f3e081f90b45e13a91505