Remove GPT-4 as the default model. (#1072)

* Remove GPT-4 as the default model.

* Updated test_compressible_agent to work around a bug that would otherwise default to gpt-4. Revist after #1073 is addressed.

* Worked around another bug in test_compressible_agent. It seems the config_list was always empty!

* Reverted changes to compressible agent.

* Noted that GPT-4 is the preferred model in the OAI_CONFIG_LIST_sample and README.

* Fixed failing tests after #1110

* Update OAI_CONFIG_LIST_sample

Co-authored-by: Chi Wang <wang.chi@microsoft.com>

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Co-authored-by: Chi Wang <wang.chi@microsoft.com>