fix: sync authotp and weighbridge custom fields via json fixtures (backport of #136)

[av-dev2]

Summary

Manual, scoped backport of #136, since the automatic backport failed with a cherry-pick conflict.

• The authotp custom fields (Customer.authotp_validated, Sales Invoice.authotp*) were only created via after_install, so any site where av_tools was already installed before these fields were added never got them.
• Moved the authotp fields and Vehicle.default_tare_weight into JSON fixtures under patches/custom_fields/custom_fields_json/, which the existing generic av_tools.utils.create_custom_fields.execute loader already runs on both after_install and after_migrate.
• Removed the now-redundant one-off patch scripts and their hook registrations, including a dead duplicate call in after_migrate.py.

Deliberately out of scope: weighbridge_ticket item-link fields

version-15/version-15-hotfix also carry a weighbridge_ticket Link field on six Item child doctypes. version-16-hotfix does not have that field — it already replaced it with a generic document_type/document_reference/target_document_type/target_document_reference design directly on Weighbridge Ticket, and tests/test_customizations.py::test_weighbridge_and_otp_custom_fields_exist explicitly asserts the old weighbridge_ticket field must not exist. Backporting the old field here would fail that test and reintroduce a superseded pattern, so this PR intentionally leaves that design alone.

Test plan

• Ran av_tools.utils.create_custom_fields.execute against a v15 test site; confirmed the authotp and Vehicle fields sync correctly via the generic loader.
• Updated test_weighbridge_and_otp_custom_fields_exist to call the generic create_custom_fields.execute loader instead of importing the now-deleted patch modules directly; the rest of that test (asserting weighbridge_ticket does not exist and the document_reference fields do) is unchanged.
• Pre-commit hooks pass on the changed files.