Remove unused dependencies

`silent-error` and `ember-cli-is-package-missing` were only used in the blueprints, which were removed in the v2 addon conversion.

Author: Windvis

packages/ember-simple-auth/package.json

@@ -32,9 +32,7 @@
     "@ember/test-waiters": "^3",
     "@embroider/addon-shim": "^1.0.0",
     "@embroider/macros": "^1.0.0",
-    "ember-cli-is-package-missing": "^1.0.0",
-    "ember-cookies": "^1.3.0",
-    "silent-error": "^1.0.0"
+    "ember-cookies": "^1.3.0"
   },
   "devDependencies": {
     "@babel/core": "7.26.9",