sla #2833 - Impossible to remove None value

With checkbox it not natural to have a "none" checkbox to check to indicate
we don't want to check anything. So I removed it.

Verified with CSV and SOAP access

Change-Id: Ic0dc394a3b02cd4e70f1007a73e66fa637287087
Merge commit 'refs/changes/60/360/2' of http://gerrit.tuleap.net/tuleap into stable

* http://gerrit.tuleap.net/tuleap:
  sla #2833 - Impossible to remove None value
This is Tuleap 5.12.99.26
clean up the changelog