Delete autogen-ext refactor deprecations

[lspinheiro]

Why are these changes needed?

The changes are

1. Delete openai client from autogen_core
2. Delete symlinks to code_executor and tools.langchain that where refactored in autogen_ext
3. Update files that relied on the deprecated imports

Those changes are needed as part of the refactor clean up before 0.4.0 release.

Related issue number

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[lspinheiro]

Seems mypy has issues with the autogen_ext imports in the docs, in this case it complains about the export type as it cant follow the import. Open to suggestions if this can be fixed rather than ignored.