[cleanup] remove unused and orphaned icons (#9123)

Removes 31 unreferenced icon assets including orphaned @2x files from prior cleanup PRs (#8231, #9036) and obsolete legacy service icons (Observatory, memory dashboard, timeline).

(Driven by `/cleanup-unused-assets` 🎉.)

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Review the contribution guidelines below:

<details>
  <summary>Contribution guidelines:</summary><br>

- See
  our [contributor guide](../CONTRIBUTING.md) and
  the [Flutter organization contributor guide]([https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md)
  for general expectations for PRs.
- Larger or significant changes should be discussed in an issue before creating a PR.
- Dart contributions to our repos should follow the [Dart style guide](https://dart.dev/guides/language/effective-dart) and use
  `dart format`.
- Java and Kotlin contributions should strive to follow Java and Kotlin best
  practices ([discussion](#8098)).

</details>

Author: pq