Remove unnecessary `package.include` field

Manually listing the files seems error-prone, and Cargo seems to be able to include the same files automatically.

See Zulip for more discussion: https://pijul.zulipchat.com/#narrow/channel/270693-general/topic/.60package.2Einclude.60.20in.20.60Cargo.2Etoml.60/with/518216974

Created by  finchie  on May 15, 2025
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