Fix Rust sui-order-signing example compile error (#14)

[pymongo]

error[E0308]: mismatched types
    --> src/order.rs:68:24
     |
68   |         bcs_bytes.push(bcs_byte);
     |                   ---- ^^^^^^^^ expected `u8`, found `u64`
     |                   |
     |                   arguments to this method are incorrect

signing-examples/rust-examples/sui-order-signing/src/order.rs

Lines 67 to 68 in b78eb39

	bcs_bytes.push(bcs_byte as u8);
	bcs_bytes.push(bcs_byte);

the line bcs_bytes.push(bcs_byte); is duplicate and redundant, I delete this line in my PR