Remove num_ctx parameter from mistral-small3.1:latest modelfile

The model file for Mistral Small 3.1 sets the number of context tokens to 4096 explicitly. This is atypical, as most other model files do not set this parameter in such a manner. Additionally, it cannot be overridden using an environment variable. Cf. #10829 for an example of the side-effects. I suggest removing that setting from the model file, in order to make behavior consistent with mistrall-small:latest (and many other model's modelfiles :) ``` # docker exec -it watchtower-ollama-1 ollama run mistral-small3.1:latest >>> /show parameters Model defined parameters: num_ctx 4096 ```