Fix write processor_subdir in serialized_dag table

[tokoko]

After #25935 SerializedDagModel.remove_deleted_dags is supposed to clean up dags only belonging to a single dag processor, but it's being ignored because processor_subdir column in serialized_dag table is never populated from dagbag in the first place.

[boring-cyborg]

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[boring-cyborg]

Awesome work, congrats on your first merged pull request! You are invited to check our Issue Tracker for additional contributions.