[FIX] project: history cumulative bad compute inherit
[FIX] project: history cumulative bad compute inherit

When we were trying to delete a stage in project where there were
some tasks, an error was raised because of the 'project.task.history.cumulative'
records that inherit 'project.task.history' which have a compute method on
on end_date.
To fix this, we set the 'compute' attribute to 'None' and the readonly
'attribute' to 'True'.

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