fix(header): remove unused variable Clippy warnings

[Himess]

This PR resolves Clippy warnings related to unused variables in core/src/header/mod.rs.

Problem

cargo clippy --all --all-targets --all-features -- -D warnings

Solution

Removed the temporary bindings and passed the expressions directly to HexDisplay(...).

Result

After this fix, cargo clippy --all --all-targets --all-features -- -D warnings runs cleanly.