Remove the unused R TypeVar from async_rpc_client.py #8876

Task Summary

amber/src/main/python/core/architecture/rpc/async_rpc_client.py defines R = TypeVar("R"), which nothing in the file or elsewhere uses. TypeVar is imported only for it.

Introduced by #2950 (2024-10-31, "Migrate scala control messages to protobuf"). The same commit expanded the typing import to bring in TypeVar.

Usage removed by: never used. No annotation, Generic[R] or string annotation in the file refers to R, and no module imports it.

This is a pure deletion with no behaviour change.

Task Type

• Refactor / Cleanup