Remove workarounds for IE that are no longer needed since IE 11 is not
supported any more.

 - Remove optional and deprecated arguments to `document.createNodeIterator`

 - Update references to IE 11 for checks/code which is still applicable
   to other browsers

 - Remove `window.msCrypto` fallback for `window.crypto`

 - Remove unused `isIE11` helper

 - Remove IE 11 reference in `polyfills/index.js`. We don't actually
   need the check because the browser check in `boot/index.js` will fail
   if `Promise` is missing, but I left it in because that knowledge is
   contained in a separate part of the code and
   `shared/polyfills/index.js` is currently written as if it may be used
   in a pre-ES2015 environment.