Tautological attr "role" in UserMessage and SystemMessage

[subramen]

As title suggests, there is a redundant attr role; this shouldn't be needed on the client since the class UserMessage and SystemMessage explicitly define the role.

Ref:

• https://github.com/meta-llama/llama-stack-client-python/blob/main/src/llama_stack_client/types/shared/system_message.py
• https://github.com/meta-llama/llama-stack-client-python/blob/main/src/llama_stack_client/types/shared/user_message.py