[FIX]Usability. While bug5213 is not fixed properly, disable at least the checkbox for admins so that they realize that emails are not to be sent when willing to let users validate their account in cases when that user account existed already beforehand.

git-svn-id: file:///var/local/code.tiki.org/tikisvn/trunk@51143 b456876b-0849-0410-b77d-98878d47e9d5