[swiftgen] clean up unnecessary ignore_for_files

[goderbauer]

Clean-up: Since FFIgen generates a blanket ignore_for_file: type=lint most of these specific ignores are not necessary. Let's remove them to avoid confusion and stop them from getting cargo-culted everywhere.

[github-actions]

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pkgs/objective_c/lib/src/ns_input_stream.dart

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[coveralls]

coverage: 81.971%. remained the same
when pulling e682d08 on goderbauer:ignore_swiftgen
into d6e6a06 on dart-lang:main.