[FIX] web: remove duplicated entry in field_registry

Commit 4960453d added the field signature in field_registry.js and
in signature.js.

closes odoo/odoo#31434

Signed-off-by: =?utf-8?b?R8OpcnkgRGVib25nbmllIChnZWQp?= <ged@openerp.com>