[BE] Move flatbuffer related python C bindings to script_init (#97476)
Summary:
Extra C binding module for flatbuffer was introduced because
not all dependencies of Pytorch want (or can) bundle in flatbuffer.
However, flatbuffer is in by default now so this separate binding is not longer needed.
Test Plan: existing unit tests
Differential Revision: D44352583
Pull Request resolved: https://github.com/pytorch/pytorch/pull/97476
Approved by: https://github.com/dbort
