[FIX] mrp: Existing operations can only be added to one BOM

Generic operations will be applied for every manufactured product passing
through the work center.

An operation linked to a bom will only be applied when the specific manufactured bom is produced

So on a BOM, every operations must be created for this specific BOM

The widget many2many is wrong because it will overwrite the generic operations

opw:2458974

closes odoo/odoo#66874

Signed-off-by: Simon Goffin (sig) <sig@openerp.com>