Python: Remove OpenAI plugin. Update OpenAPI exec params to use direc…

…t predicate reference (#10095)

### Motivation and Context

Dotnet had long removed the OpenAI plugin from its code base, and Python
did not follow at the time. We had previously provided deprecation
messages around using the OpenAI plugin and to switch to the OpenAPI
plugin. We are now removing the OpenAI plugin from the SK Python SDK.

Additionally, there was a forward reference created without specifying
the direct predicate context reference in the
OpenAPIFunctionExecutionParameters. This put the burden on the dev to
have to perform a model rebuild once the import was added. This PR
cleans up the `OperationSelectionPredicateContext` import, which is now
direct and via the sub-module and not via the parent module, which
causes the __init__.py to load all dependencies (which is unnecessary).

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### Description

This PR:
- removes the unsupported OpenAI plugin (previously marked as
deprecated)
- fixes the `OperationSelectionPredicateContext` import to allow a user
to create the func exec params without receiving a pydantic error.
- updates unit tests to remove references to OpenAI plugin.

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### Contribution Checklist

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- [X] The code builds clean without any errors or warnings
- [X] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [X] All unit tests pass, and I have added new tests where possible
- [ ] I didn't break anyone 😄