Don't clone before borrowing

Clippy told us to change function parameters to references, but
wasn't able to then tell us that the clone() before borrowing
was superfluous. This patch removes those by hand.

No expectation of a performance improvement here, since we were
just cloning reference counts. Just removes a bunch of noise.

benches/bank.rs

@@ -32,7 +32,7 @@ fn bench_process_transaction(bencher: &mut Bencher) {
 
             let rando1 = KeyPair::new();
             let tx = Transaction::new(&rando0, rando1.pubkey(), 1, last_id);
-            assert!(bank.process_transaction(&tx.clone()).is_ok());
+            assert!(bank.process_transaction(&tx).is_ok());
 
             // Finally, return the transaction to the benchmark.
             tx