crimson/tools/store_nbd: drop unnecessary seastar::now()

the body of handle_exception() is a synchronous operation, there is no
need to return seastar::now() here.

Signed-off-by: Kefu Chai <kchai@redhat.com>

src/crimson/tools/store_nbd/store-nbd.cc

@@ -449,7 +449,6 @@ void NBDHandler::run()
                                        output.close());
             }).discard_result().handle_exception([](auto e) {
               logger().error("NBDHandler::run saw exception {}", e);
-              return seastar::now();
             });
           });
       });