Remove from the 2.x API tf.train.Saver and related symbols which rely on sessions/collections

Saver will be replaced by tf.train.Checkpoint (and tf.contrib.checkpoint.CheckpointManager) for training checkpoints, and by a simple Python representation of a SaverDef (which may not be a public symbol).

tf.train.Checkpoint does not write/merge sharded checkpoints at the moment, so v2 will want a solution for that (tf.train.ShardedCheckpoint?).

MetaGraph import and export will be replaced by object-based tf.saved_model.import/tf.saved_model.export.

PiperOrigin-RevId: 218262301