Remove Longformers from ONNX-supported models
# What does this PR do?

This PR removes Longformer from the list of ONNX-supported models due to a current limitation in the supported ops in JIT - see issue #15217  

This slipped through our unit tests because we didn't include `longformer` in the list of models to be tested.

Long term, we'll either need to wait for JIT to support the problematic ops or tweak the Longformer implementation.

_The documentation is not available anymore as the PR was closed or merged._