BC Fix Stage 2 fully migrate to constant data (#1960)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1960

This is part 2 of stage 2. We are updating the model generation to generate XN01 models. The change here is removing all usage of old constant data serialization (serializing data in flatbuffer) and moving completely to serializing data using the XNNHeader. This removes a few flatbuffer constructs like Buffer and also removes items from the XNNGraph like constant_buffer and mem_id_sizes

Reviewed By: digantdesai, kirklandsign

Differential Revision: D53495025

fbshipit-source-id: 23211d92a690daf8e439ddf46547f966befe96ba