[various] Remove CocoaPods from examples (#11237)

For almost all projects in the repository, this does the steps recommended by the warning when building with Swift Package Manager enabled (for both ios/ and macos/, wherever they are present):
- Runs `pod deintigrate`
- Deletes `Podfile`
- Removes the CocoaPod xcconfig references in the Flutter Debug and Release xcconfigs.

When running without Swift Package Manager enabled, `flutter` will automatically re-add this integration, so this won't break anyone running these examples on `stable`. However, we are no longer expecting this to be a common workflow (CI no longer builds these apps in non-SwiftPM mode, and regular flutter/package contributors are likely to be on `master` where SwiftPM is on by default now), so it's no longer useful to have the integration checked in.

The only packages that have not had Pod integration removed are:
- `google_maps_flutter_ios` because it doesn't (and can't) support Swift Package Manager.
- `google_maps_flutter` because it depends on `google_maps_flutter_ios` as the default implementation.

This also removes the precache steps from `ios_build_all_packages` and `macos_build_all_packages`. Because that step runs per package, it was doing a lot of caching of native code that was package-scoped, and thus wouldn't be used by the all-packages app, and in the current configuration isn't even the right kind of dependency (it will fetch SwiftPM deps, but the all-packages app uses CocoaPods still to cover that mode).

Follow-up to https://github.com/flutter/flutter/issues/172427

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