remove empty page_action (#1517)

Author: hogashi

src/extension/chrome/manifest.json

@@ -8,7 +8,6 @@
     "128": "images/logo128.png",
     "400": "images/logo400.png"
   },
-  "page_action": {},
   "devtools_page": "devtools.html",
   "background": { "service_worker": "service_worker.js" },
   "web_accessible_resources": [