Remove Chrome support

@rebloor [noted][1] that the root readme states that "examples are tested in Firefox." This change removes Chrome support in order to accurately reflect the framing established in the README.

[1]: https://github.com/mdn/webextensions-examples/pull/565#issuecomment-2241954087

stored-credentials/manifest.json

@@ -15,8 +15,7 @@
   },
 
   "background": {
-    "scripts": ["storage.js", "auth.js"],
-    "service_worker": "sw.js"
+    "scripts": ["storage.js", "auth.js"]
   },
 
   "options_ui": {

stored-credentials/sw.js

@@ -1,2 +0,0 @@
-// Import and synchronously execute other JavaScript files.
-importScripts("storage.js", "auth.js");