Remove unused PhantomJS dependency (preactjs#323)

Author: 38elements

package.json

@@ -75,7 +75,6 @@
     "mkdirp": "^0.5.1",
     "mocha": "^5.2.0",
     "npm-run-all": "^3.0.0",
-    "phantomjs-prebuilt": "^2.1.7",
     "preact": "^10.0.0-beta.0",
     "pretty-bytes-cli": "^1.0.0",
     "puppeteer": "^1.9.0",