Remove premium tax credit code from IRS section

I believe this is duplicative of the `gov/aca` code that @martinholmer created.

https://github.com/PolicyEngine/policyengine-us/tree/master/policyengine_us/parameters/gov/irs/credits/premium_tax_credit

Compare policyengine_us/parameters/gov/irs/credits/premium_tax_credit with the corresponding gov/aca code created by @martinholmer, then inspect references to determine whether the IRS section is duplicated. Done means the redundant IRS code is removed without leaving broken references or changing the intended premium tax credit behavior.

Written by the indexing model from the issue text.