API: disable the venice system prompt by default

The venice.ai API injects the venice system prompt by default.

That's a bit unexpected and wastes tokens.

The API should be raw, it is usually used by agents and other systems that bring their own system prompts.

Please disable the venice system prompt by default in the API or at least add a global or per-key option to disable it because it is very inconvenient to add a custom request parameter to each API consumer.

For reference: https://docs.venice.ai/api-reference/endpoint/chat/completions#body-venice-parameters