[tf.data] Remove the `Dataset.output_{types,shapes,classes}` properties in V2.

These reflection properties are lightly used in non-test code, and do
not properly reflect some of the recent developments in custom and
nested structures (e.g. nested datasets returned by
`Dataset.window()`, datasets containing `tf.data.experimental.Optional`, etc.).

For internal users,
`dataset_ops.get_legacy_output_{types,shapes,classes}(dataset)`
provide a short-term replacement, and `dataset_ops.get_structure(dataset)`
exposes the same information as a `tf.data.experimental.Structure` object.

The replacement public property/API is still TBD, and will be added
post 2.0-alpha.

PiperOrigin-RevId: 235621094