remove unused which dependency

Author: Kelly Selden

package.json

@@ -49,7 +49,6 @@
     "semver": "^6.0.0",
     "tmp": "0.1.0",
     "update-notifier": "^3.0.0",
-    "which": "^1.3.1",
     "yargs": "^14.0.0"
   },
   "devDependencies": {