Make it more clear that JavaScript is disabled on security level "high"

If a user is setting the security slider level to "high" and is reading its description, the first thing JavaScript related that shows up is: "All JavaScript performance optimizations are disabled. Scripts on some sites my run slower."

This is pretty confusing as a bit later it is pointed out that "JavaScript is disabled by default on all sites."

We should remove the first explanation on this security level. Thanks for karsten pointing this out.