Fix warning in memory-benchmark.hs.

```
bench/memory-benchmark.hs:153:22: error: [-Wunused-matches, -Werror=unused-matches]
    Defined but not used: `tr'
    |
153 | getLatestBlockHeight tr wallet = do
```

lib/wallet-benchmarks/bench/memory-benchmark.hs

@@ -150,7 +150,7 @@ copyNodeSnapshot snapshot tmp = do
     Cardano commands
 ------------------------------------------------------------------------------}
 getLatestBlockHeight :: Tracer IO Text -> C.CardanoWalletConn -> IO Int
-getLatestBlockHeight tr wallet = do
+getLatestBlockHeight _tr wallet = do
     fmap (fromMaybe 0 . readMaybe)
         . flip S.readCreateProcess ""
         . S.shell