[material_ui] Remove unused parameters from constructors of generic classes.

[srawlins]

We already don't allow unused parameters from non-generic classes. There was a bug preventing us from reporting such parameters on generic classes.

This is work towards dart-lang/sdk#47839.

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
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• 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.
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• All existing and new tests are passing.

Test exempt, as this code is covered by existing static and runtime checks.

[gemini-code-assist]

Code Review

This pull request removes unused super.key parameters from several private widget constructors across the material_ui package, including _AutocompleteOptions, _ModalBottomSheet, _DropdownMenuItemButton, _DropdownMenu, _DropdownRoutePage, _MenuItem, _PopupMenu, and _SegmentedButtonRenderWidget. Additionally, it removes the menuWidth and underline parameters from certain dropdown constructors, initializing them to null directly, and simplifies the _CustomPageRoute constructor in tests. There are no review comments, and I have no feedback to provide.