Remove Fireworks/Qwen support for SaaS

Since the feature flag fireworks_qwen_code_completion is currently disabled on gitlab.com and we do not have plans to turn it back on, we should remove the feature flag code from Gitlab/Rails to simplify the code completion model selection process.

This would also remove the code_completion_model_opt_out_from_fireworks_qwen feature flag as it would no longer be necessary.

It would still be possible for self-hosted users to use Fireworks/Qwen in previous relases, so support in the AIGW should remain. It also could be selected by the user/group once code completion model selection is implemented.