Keep only adds or formula in household_benefits

[MaxGhenis]

As required by new policyengine-core, this has a formula and adds

policyengine-uk/policyengine_uk/variables/household/income/benefit.py

Line 52 in b9f48c1

class household_benefits(Variable):

[anth-volk]

Also

policyengine-uk/policyengine_uk/variables/gov/hmrc/tax.py

Line 14 in b9f48c1

class household_tax(Variable):

[MaxGhenis]

Let's use adds and reforms as needed. Switching to a formula would result in it not decomposing in the app. See PolicyEngine/policyengine-us#3851

[MaxGhenis]

See PolicyEngine/policyengine-us#3972 for an example of a PR that applies reforms for these conditional lists