Fix attendance interpose

Validating attendance by attendance generate a lot of errors
in case you interpose attendance between other.
So I remove this check in the constraint and set the validation
into the confirm button of the timesheet.
To test all modules together except those two, and then individually test each of the modules in the repo, you can:

    - VERSION="7.0" ODOO_REPO="odoo/odoo" EXCLUDE="hr_timesheet_improvement,hr_attendance_analysis"
    - VERSION="7.0" ODOO_REPO="odoo/odoo" UNIT_TEST="1"
    - VERSION="7.0" ODOO_REPO="OCA/OCB" EXCLUDE="hr_timesheet_improvement,hr_attendance_analysis"
    - VERSION="7.0" ODOO_REPO="OCA/OCB" UNIT_TEST="1"