[camera] Remove outdated TODO in camera_controller.dart

[cdeil]

@stuartmorgan-g - this PR is a small cleanup that resolves an old TODO comment referencing Dart 2.
The camera package now requires Dart 3.9 so unawaited can be used.

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[flutter-dashboard]

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[gemini-code-assist]

Code Review

This pull request removes a local _unawaited helper function and its associated TODO comment in packages/camera/camera/lib/src/camera_controller.dart. All usages of _unawaited have been replaced with the unawaited function from dart:async.

[stuartmorgan-g]

LGTM

[stuartmorgan-g]

Version/changelog override: while this technically changes production code, it is a very straightforward no-op.