Perform GA-ready scrub (experimental/deprecated cleanup)

[garrettjonesgoogle]

• Consider whether google-cloud-java should have its own definition of @ExperimentalApi (or maybe it should be in gax-java). Protobuf has one (public), and gRPC has its own of the same name (marked @Internal).
• In google-cloud-core, mark everything that is not used by Datastore/Storage/Logging as experimental.
• Remove any remaining @Deprecated things
• Make sure no deprecated features in dependencies are used that have a replacement

[garrettjonesgoogle]

Known usage of @Deprecated that needs to be updated: googleapis/gax-java#141 ("Use MoreCallCredentials.from for auth instead of ClientAuthInterceptor which is deprecated")

[garrettjonesgoogle]

All deprecated and experimental usages from Datastore/Storage/Logging outside of tests have been removed. The only caveat is that googleapis/gax-java#141 isn't pulled into google-cloud-java through a gax release yet, but it will be pulled in later.

All features in core not used by Datastore/Storage/Logging is marked @BetaApi.

All @Deprecated things have been removed.

Closing issue.