Sub-task should not allow to set issue-level security because Subtaska doesn't have independent security levels.

Subtasks inherit the security level of their parent issue, so we need to block the option to set the security level into a sub-task.

Despite of this set the security level into the sub-task, when I refresh the page this will be removed. 

Besides, this feature request: Sub-Task should have independent security levels. was closed as "Won't Fix". As we will not implement the security level independency to sub-task, we can show a message when anyone tries to set it into a sub-task.