Simplify lookup in radio conditional reveal sync

Use document.getElementById rather than document.querySelector which requires us to manually prepend the hash to get an ID selector.

src/govuk/components/radios/radios.js

@@ -77,7 +77,7 @@ Radios.prototype.syncAllConditionalReveals = function () {
  * @param {HTMLInputElement} $input Radio input
  */
 Radios.prototype.syncConditionalRevealWithInputState = function ($input) {
-  var $target = document.querySelector('#' + $input.getAttribute('aria-controls'))
+  var $target = document.getElementById($input.getAttribute('aria-controls'))
 
   if ($target && $target.classList.contains('govuk-radios__conditional')) {
     var inputIsChecked = $input.checked