Remove the Flutter outline in Q1 2025

In Q4 2024, this was added (https://github.com/flutter/flutter-intellij/commit/d9c57873b6eecc38dd06e1a6d06385e9c9264354, https://github.com/flutter/flutter-intellij/commit/86a3f06f1a25ccc343da02ebc6c3a98e3e6b83d2) to warn users that the Flutter Outline was going away.  In Q1 2025 the Outline window in the Flutter IJ plugin should be deleted.