[go_router] Remove unused dependency (#4545)

Removes `adaptive_dialog` from the example pubspec, as it is not used, and removes the repository allowance for it.

Also updates the comments for the remaining pinned dependency exceptions to reflect the results of the audit.

Part of https://github.com/flutter/flutter/issues/130897

packages/go_router/example/pubspec.yaml

@@ -8,7 +8,6 @@ environment:
   flutter: ">=3.3.0"
 
 dependencies:
-  adaptive_dialog: 1.8.2
   adaptive_navigation: ^0.0.4
   collection: ^1.15.0
   cupertino_icons: ^1.0.2

script/configs/allowed_pinned_deps.yaml

@@ -4,14 +4,16 @@
 # All entries here should have an explanation for why they are here.
 
 # TODO(stuartmorgan): Eliminate this in favor of standardizing on
-# mockito.
+# mockito. See https://github.com/flutter/flutter/issues/130757
 - mocktail
 
-# Legacy allowances, for dependencies that predate the tooling.
-# TODO(stuartmorgan): Audit these. See
-# https://github.com/flutter/flutter/issues/122713
+# Test-only dependency, so does not impact package clients, and
+# has limited impact so could be easily removed if there are
+# ever maintenance issues in the future.
 - lcov_parser
-- adaptive_dialog
+
+# This should be removed; see
+# https://github.com/flutter/flutter/issues/130897
 - provider
 
 ## Allowed by default