removed empty background.js (#897)

functional-samples/reference.mv3-content-scripts/manifest.json

@@ -2,9 +2,6 @@
   "name": "MV3 Migration - content script example",
   "version": "0.1",
   "manifest_version": 3,
-  "background": {
-    "service_worker": "background.js"
-  },
   "permissions": ["scripting", "activeTab"],
   "action": {
     "default_popup": "popup.html"