Remove the deprecated `PrivacyDeclaration.dataset_references` and `System.system_dependencies` fields

[PSalant726]

The dataset_references field on PrivacyDeclaration resources and the system_dependencies field on System resources were deprecated in #85 (see https://github.com/ethyca/fideslang/pull/85/commits/36638c3c4a3cbd2bba80a291e779970b9d2434b3 and https://github.com/ethyca/fideslang/pull/85/commits/6e03b27021f953a8a883bf0e5f116d355b824a5b). These fields should be removed.