Remove `tests/onnx`
# What does this PR do?

Remove `tests/onnx`, as mentioned, as discussed

https://github.com/huggingface/transformers/pull/24800#issuecomment-1634822781

Note there are still some tests like `TFGPT2ModelTest::test_onnx_runtime_optimize` which are not removed in this PR.
_The documentation is not available anymore as the PR was closed or merged._