remove opn-cli dev dependency as it has been replaced by karma

package.json

@@ -97,7 +97,6 @@
     "mocha-multi-reporters": "^1.1.7",
     "npm-run-all": "^4.1.5",
     "nyc": "^14.1.1",
-    "opn-cli": "^5.0.0",
     "rollup": "^1.16.6",
     "rollup-plugin-alias": "^1.5.2",
     "rollup-plugin-commonjs": "^10.0.1",
@@ -171,4 +170,4 @@
     ],
     "all": true
   }
-}
+}