refactor: compute n_copies once

Co-authored-by: Carol (Nichols || Goulding) <193874+carols10cents@users.noreply.github.com>

router/src/dml_handlers/rpc_write.rs

@@ -159,7 +159,7 @@ impl<T> RpcWrite<T> {
 
         Self {
             endpoints,
-            n_copies: replica_copies.map(NonZeroUsize::get).unwrap_or(1),
+            n_copies,
         }
     }
 }