Mark tf.summary v1 API classes and tf.train.summary_iterator() as v1-only

The tf.summary.FileWriter and FileWriterCache classes are only for v1 summary writing; TF 2.0 provides the v2 create_file_writer() API instead.

The tf.summary python protos are being removed as part of the overall effort to remove protos from the TF 2.0 python API. The recourse for users will be accessing these bindings from within TensorBoard - the exact package path is still TBD, but when we have one we can add a migration hint.

As for tf.train.summary_iterator(), this can be replaced by just using tf.io.record_iterator() and parsing each record with Event.FromString() using the Event proto symbol, wherever in TensorBoard it ends up.

PiperOrigin-RevId: 228564320