Revert "[IMP] calendar: Set default value for start* and stop* date/datetime. Else field.function start and stop are not setted by default while they are required."

This reverts commit b1adf3a95e94568e618d635ce575398e94c3967e.

Not so simple, 'search' don't find recurrent events with this change
Merge branch '8.0' of https://github.com/odoo/odoo into 8.0-win32