Remove result handling for eCAL_Subscriber_SetReceiveCallback

[LeWimbes]

Summary

Split out of #69 - this PR contains only the API‑compatibility fix.

Since eCAL 6.0.0-rc.4 eCAL_Subscriber_SetReceiveCallback returns void instead of an int.
With this PR, the return value is no longer expected in subscriber.rs.

Checklist

• I have tested this change locally
• I have documented any public APIs or CLI changes
• I have added appropriate examples or comments
• The code builds and passes all checks (cargo check, cargo test)
• I have updated the changelog if applicable