Remove workaround for ruma/ruma#1940

Author: andybalaam

src/identifiers.rs

@@ -120,16 +120,7 @@ impl DeviceKeyId {
     /// Returns device ID of the device key ID.
     #[wasm_bindgen(getter, js_name = "deviceId")]
     pub fn device_id(&self) -> DeviceId {
-        // TODO: when https://github.com/ruma/ruma/issues/1940 is fixed,
-        // this should just be:
-        //self.inner.key_name().to_owned().into()
-
-        let key_id = self.inner.to_string();
-
-        let colon_pos =
-            key_id.find(":").expect("Key should not have parsed if it did not contain ':'");
-
-        DeviceId::new(&key_id[(colon_pos + 1)..])
+        self.inner.key_name().to_owned().into()
     }
 
     /// Return the device key ID as a string.