Fixes a few issues in `jsvat`:
- Since the end of 2022, valid Belgian VAT numbers starting with `BE1`
have been attributed, the regex had to be adjusted.
- Since Brexit, Northern Ireland has to use the `XI` prefix instead of
`GB`.
- Removes the default country code logic. As Odoo is used worldwide, it
didn't make sense to use `GB` as the default country code when no
country code is mentioned.
closesodoo/odoo#139098
X-original-commit: f486a41f729ea7e934c81671972b3b1162986860
Signed-off-by: Francois Georis (fge) <fge@odoo.com>
Signed-off-by: Louis Baudoux (lba) <lba@odoo.com>