Audit whether council_tax_less_benefit is dead code

[MaxGhenis]

After #1668 fixes the net income tree to include council_tax_benefit directly, the standalone council_tax_less_benefit variable (variables/household/consumption/council_tax_less_benefit.py) is no longer consumed inside the model. Grep:

$ grep -rln "council_tax_less_benefit" policyengine_uk/
policyengine_uk/variables/household/consumption/council_tax_less_benefit.py

Only definition, no consumers in policyengine_uk/. External consumers (e.g. uk-land-value-tax, dashboards) may still use it for display purposes — it's a convenient pre-computed gross-minus-CTR for tables and charts. Worth confirming before removing.

Options:

1. Keep as a public-facing convenience variable, document it as such.
2. Remove if no external repo consumes it.

Found while reviewing #1668.