Revert "Config objects as Pydantic BaseModels (#6176)"

[tofarr]

This reverts commit 873dddb.

I get the following when using the remote runtime:

ERROR:asyncio:Task exception was never retrieved
future: <Task finished name='Task-39' coro=<Session.initialize_agent() done, defined at /Users/tofarr/dev/all-hands/OpenHands/openhands/server/session/session.py:66> exception=AttributeError("'str' object has no attribute 'get_secret_value'")>
Traceback (most recent call last):
  File "/Users/tofarr/dev/all-hands/OpenHands/openhands/server/session/session.py", line 93, in initialize_agent
    llm = LLM(config=self.config.get_llm_config_from_agent(agent_cls))
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/tofarr/dev/all-hands/OpenHands/openhands/llm/llm.py", line 117, in __init__
    self.init_model_info()
  File "/Users/tofarr/dev/all-hands/OpenHands/openhands/llm/llm.py", line 324, in init_model_info
    'Authorization': f'Bearer {self.config.api_key.get_secret_value() if self.config.api_key else None}'

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To run this PR locally, use the following command:

docker run -it --rm   -p 3000:3000   -v /var/run/docker.sock:/var/run/docker.sock   --add-host host.docker.internal:host-gateway   -e SANDBOX_RUNTIME_CONTAINER_IMAGE=docker.all-hands.dev/all-hands-ai/runtime:504782d-nikolaik   --name openhands-app-504782d   docker.all-hands.dev/all-hands-ai/openhands:504782d