Remove ONNX (#146325)

Summary:
ONNX exporter experiments in benchmark is obsolete and unmaintained. This PR removes it to unblock pytorch/pytorch#146003

X-link: pytorch/pytorch#146325
Approved by: https://github.com/titaiwangms

Reviewed By: ZainRizvi

Differential Revision: D69115633

fbshipit-source-id: 20f264458f38d1dafde100970f07b6622e5b3d6e

Author: generatedunixname499836121