chore: remove unused dev packages (#334)

Author: JustinBeckwith

packages/google-cloud-oslogin/package.json

@@ -59,7 +59,6 @@
     "mocha": "^7.0.0",
     "null-loader": "^3.0.0",
     "pack-n-play": "^1.0.0-2",
-    "prettier": "^1.11.1",
     "sinon": "^9.0.1",
     "ts-loader": "^6.2.1",
     "typescript": "^3.8.3",