[FIX] website_sale_aplicoop: Remove redundant string= attributes and fix OCA linting warnings

- Remove redundant string= from 17 field definitions where name matches string value (W8113)
- Convert @staticmethod to instance methods in selection methods for proper self.env._() access
- Fix W8161 (prefer-env-translation) by using self.env._() instead of standalone _()
- Fix W8301/W8115 (translation-not-lazy) by proper placement of % interpolation outside self.env._()
- Remove unused imports of odoo._ from group_order.py and sale_order_extension.py
- All OCA linting warnings in website_sale_aplicoop main models are now resolved

Changes:
- website_sale_aplicoop/models/group_order.py: 21 field definitions cleaned
- website_sale_aplicoop/models/sale_order_extension.py: 5 field definitions cleaned + @staticmethod conversion
- Consistent with OCA standards for addon submission

website_sale_aplicoop/readme/CREDITS.md

@@ -6,4 +6,3 @@ The implementation follows OCA standards for:
 - Code quality and testing (26 passing tests)
 - Documentation structure and multilingual support
 - Security and access control
-

website_sale_aplicoop/readme/USAGE.md

@@ -48,4 +48,3 @@
 
 - `start_date` must be ≤ `end_date` (when both filled)
 - Empty end_date = permanent order
-