refactor: remove unused payload field from enum

[hamirmahal]

closes #118.

[hamirmahal]

These commands succeeded when I tested this change locally.

cargo build --workspace --all-features
cargo test --workspace --all-features --no-run
cargo test --workspace --all-features
cargo build --workspace --all-features --examples
cargo test --workspace --all-features --examples --no-run
cargo test --workspace --all-features --examples
cargo clippy --all --all-features

[obmarg]

Thanks for the contribution @hamirmahal - I'm not sure that removing those fields is the right fix though. Potentially should be exposing those fields to the user somehow rather than just removing them...

At the very least that's the case for the ConnectionAck one - and maybe the ping/pong ones as well (although I can see less obvious use cases for those ones).