Resolve "Remove Fireworks/Qwen support for SaaS"

What does this MR do and why?

Removes the following feature flags fireworks_qwen_code_completion and code_completion_model_opt_out_from_fireworks_qwen - Removes Fireworks/Qwen support for Saas as this has been replaced by Codestral 25.01 disable_code_gecko_default - this has already been removed on the AIGW and no longer has any effect, so this is simply cleanup.

Removing these feature flags simplifies our model selection code and makes it less error prone as we move forward with new features based on model selection

References

How to set up and validate locally

1. Code completion should work as expected

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Related to #525573 (closed)