We don't allow deleting a budget with associated investments. However,
we allow deleting a budget with associated administrators and valuators.
This results in a foreign key violation error:
PG::ForeignKeyViolation: ERROR: update or delete on table "budgets"
violates foreign key constraint "fk_rails_c847a52b1d" on table
"budget_administrators"
Using the `dependent: :destroy` option when defining the relationship,
we remove the association records when removing the budget.
As a bonus, we reduce the number of Rubocop offenses regarding the
`Rails/HasManyOrHasOneDependent` rule. Only 72 to go! :)
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