[IMP] web_calendar: Remove one2one reference
[IMP] web_calendar: Remove one2one reference

This one2one reference is no longer needed since one2one fields have been
deprecated for ages

Closes #10746

ygol/odoo,ygol/odoo,dfang/odoo,dfang/odoo,ygol/odoo,ygol/odoo,hip-odoo/odoo,dfang/odoo,hip-odoo/odoo,ygol/odoo,hip-odoo/odoo,dfang/odoo,hip-odoo/odoo,hip-odoo/odoo,dfang/odoo,ygol/odoo,dfang/odoo,hip-odoo/odoo,ygol/odoo