infra: temporarily drop OpenAI from core release test matrix (#31693)

As part of core releases we run tests on the last released version of
some packages (including langchain-openai) using the new version of
langchain-core. We run langchain-openai's test suite as it was when it
was last released.

Our test for computer use started raising 500 error at some point during
the day today (test passed as part of scheduled test job in the
morning):
> InternalServerError: Error code: 500 - {'error': {'message': 'An error
occurred while processing your request. You can retry your request, or
contact us through our help center at help.openai.com if the error
persists.

Will revert this change after we release langchain-core.