dbus: networkmanager: do not "use async_std" as it is already in scope

We can just use dependencies listed in Cargo.toml under their name without
having to explicitly "use" them.

Signed-off-by: Leonard Göhrs <l.goehrs@pengutronix.de>

src/dbus/networkmanager.rs

@@ -16,7 +16,6 @@
 // 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
 
 use anyhow::Result;
-use async_std;
 use async_std::sync::Arc;
 use serde::{Deserialize, Serialize};