Customer statements + late-fee billing now live in the standalone
`customer_statements` app, so they can be versioned and deployed
independently of the NMI payment integration.
Removed here: api/statements.py, setup.py (its custom fields existed only
for late fees), customer_statements.js, customer_list.js, the statement
template, and the feature doc. hooks.py drops the statements JS,
doctype_list_js, and after_migrate accordingly.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>