[CQ] remove FlutterCreateAdditionalSettingsField deadcode

[pq]

Following up on a TODO from Steve to remove an unused method and callers.

Interestingly, FlutterModuleGroup seems entirely unused. It's meant to:

// Define a group of Flutter project types for use in Android Studio 4.2 and later.

Steve warns:

// TODO (messick) DO NOT delete this during post-4.2 clean-up.

but maybe we can? Especially if noone has plans to re-integrate.

@jwren : do you have any context on this?

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[jwren]

The change LGTM, but do build and test it on both Meerkat and Narwhal before landing, smaller changes than this have caused a number of issues with this project.