Properly disable everything related to E4X when E4X is turned off

[p-bakker]

At least the for each loops are not disabled when the E4X feature flag is turned off:

There might be more stuff that is part of E4X that ought to throw a SyntaxError when used while E4X is disabled

[p-bakker]

for each (var x in ...) was part of JavaScript 1.6 (that introduced E4X as well) and got deprecated as part of deprecating E4X as well"

• https://web.archive.org/web/20201028140805/https://developer.mozilla.org/en-US/docs/Archive/Web/JavaScript/New_in_JavaScript/1.6
• https://web.archive.org/web/20201027041511/https://developer.mozilla.org/en-US/docs/Archive/Web/JavaScript/for_each...in