[material_ui] Remove unused example files

[dkwingsmt]

These example files are not used in material_ui anywhere. They're testing widgets defined in flutter/widgets, and these examples are only referenced in docs in flutter/widgets.

This PR removes these files from material_ui. A subsequent PR will fix flutter/widgets to move these example files to flutter/widgets/examples, preserving them after the deprecation of flutter/material.

There are no cases of the same kind in cupertino_ui.

Note to reviewers: This is not a blocker to the 1.0 release, since it's a no-op. I'm happy to postpone this PR if deemed necessary.

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the AI contribution guidelines and understand my responsibilities, or I am not using AI tools.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter.
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to at least one issue that this PR fixes in the description above.
• I followed the version and CHANGELOG instructions, using semantic versioning and the repository CHANGELOG style, or I have commented below to indicate which documented exception this PR falls under¹.
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under¹.
• All existing and new tests are passing.

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Footnotes

1. Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling. ↩ ↩²

[gemini-code-assist]

Code Review

This pull request removes multiple example files and their corresponding widget tests related to context menus, platform menu bars, and selection containers. Feedback indicates that deleting editable_text_toolbar_builder.2.dart will break an existing documentation reference in adaptive_text_selection_toolbar.dart, which must be updated or removed to prevent build failures.

[dkwingsmt]

/gemini review

[gemini-code-assist]

Code Review

This pull request removes several example files and their corresponding widget tests related to context menus, platform menu bars, selectable regions, and selection containers. I have no feedback to provide as there are no review comments.