Remove platforms: default_package in pubspec.yaml

[xvrh]

Latest Flutter version from the beta channel (v3.25.0) has a warning with this entries in the pubspec.yaml

Package open_filex:linux references dartPluginClass:linux as the default plugin, but the package does not exist.
Ask the maintainers of open_filex to either avoid referencing a default implementation via `platforms: linux: default_package: dartPluginClass` or create a plugin named dartPluginClass.

I think this is just not needed.