Remove allowance variables that are only parameters

[MaxGhenis]

trading_allowance, property_allowance, and dividend_allowance are currently just parameters, e.g.:

https://github.com/PolicyEngine/openfisca-uk/blob/0351faef15927b037de9600eac529510b3770965/openfisca_uk/variables/finance/tax/income_tax/allowances.py#L79