refactor(rooch-benchmarks): simplify tx building logic

Removed unnecessary intermediate steps by directly returning the result of `build_and_sign`. This enhances code readability and reduces redundancy.

crates/rooch-benchmarks/src/tx.rs

@@ -64,9 +64,7 @@ pub fn create_l2_tx(
         _ => panic!("Unsupported tx type"),
     };
 
-    let mut tx = test_transaction_builder.build_and_sign(action)?;
-    let _ = tx.tx_hash();
-    Ok(tx)
+    test_transaction_builder.build_and_sign(action)
 }
 
 pub fn find_block_height(dir: &Path) -> Result<Vec<u64>> {