[ffigen] Remove most of the Config extension

[liamappelbe]

Working towards removing the Config extension type. This PR removes all the methods that can be trivially removed.

Not touching importedTypesByUsr, typedefTypeMappings, structTypeMappings, unionTypeMappings, or importedIntegers yet. They're a bit more involved, so I'll do those in a separate PR.

[coveralls]

coverage: 82.352% (+0.4%) from 81.98%
when pulling c221448 on ffigen_config_cleanup_4
into dac4797 on main.

[github-actions]

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pkgs/hooks_runner/test_data/download_assets/hook/build.dart
pkgs/objective_c/example/command_line/lib/main.dart
pkgs/objective_c/lib/src/ns_input_stream.dart

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