Remove `s_corp_self_employment_income` variable and update SE tax base

[MaxGhenis]

Currently we levy self-employment taxes on s_corp_self_employment_income:

policyengine-us/policyengine_us/variables/gov/irs/tax/self_employment/taxable_self_employment_income.py

Lines 13 to 17 in 285aa5a

	SEI_SOURCES = [
	"self_employment_income",
	"farm_income",
	"s_corp_self_employment_income",
	]

This is incorrect - S-corp distributions are not subject to SE tax. The variable is also not used elsewhere in the code, and its description suggesting it's included in partnership_s_corp_income is wrong, as that variable has no formula and comes straight from the PUF.

Changes needed:

1. Remove s_corp_self_employment_income variable
2. Update taxable_self_employment_income formula to use only:
   • Schedule C income (self_employment_income)
   • Schedule F income (farm_income)
   • Partnership portion of distributions (to be added, see related issue in policyengine-us-data)