Bug 35718: Remove ES6 warns from JS system preferences

Bug 35718: Remove ES6 warns from JS system preferences

To test:
1. Add some JS like this to any of the UserJS system preferences:

$(document).ready( function() {
    let something = 1;
    const another_thing = 2;

    let an_arrow_function = (a, b) => a + b;
    console.log( an_arrow_function(something, another_thing) );
});

2. Notice the icons and warnings to the left of the line numbers:

let is available in ES6
const is available in ES6
arrow_function_syntax is available in ES6

3. APPLY PATCH
4. Try steps 1 and 2 again, the warnings should be gone.
5. Check that the JavaScript still works, in my example it should console.log 3.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>