Clean up unneeded dependency

packages/attachments/package.json

@@ -34,7 +34,6 @@
     "@jupyterlab/observables": "^1.0.9",
     "@jupyterlab/rendermime": "^0.16.2",
     "@jupyterlab/rendermime-interfaces": "^1.0.9",
-    "@phosphor/algorithm": "^1.1.2",
     "@phosphor/disposable": "^1.1.2",
     "@phosphor/signaling": "^1.2.2"
   },