[hooks_runner] Remove package_config_hashable.json workaround

[dcharkes]

Newer pub versions don't seem to add a timestamp under the generated key anymore.

• Don't rewrite package_config and package_graph if unchanged pub#4549

So we can remove the workaround that was added in:

• [native_assets_builder] Fix hook compile caching  #1933

This would also make the reported dependencies more useful for Flutter. (E.g. reporting a dependency on package_config_hashable.json which is a copy of the original one makes flutter ignore the actual package_config.json changes when reinvoking hooks!)