[configuration] Fix option deletion issue. (#9121)

The handler in configuration/jsx/configuration_helper.js was not properly taking the option to be deleted since the fields not always have necessary to be of SELECT type (they could be also text for example).
This updates the code so it should take the appropriate option to be deleted.

Fixes #9096

modules/configuration/jsx/configuration_helper.js

@@ -39,11 +39,9 @@ $(function() {
     $('.btn-remove').on('click', function(e) {
         e.preventDefault();
 
-        let options = $(this).parent().parent().children().prop('options');
-        let selectedIndex = $(this)
-                            .parent().parent().children()
-                            .prop('selectedIndex');
-        let selectedOption = options[selectedIndex].text;
+        let selectedOption = $(this).parent().parent().children()
+                             .prop('value');
+
         let fieldName = $(this)
                         .parent().parent().parent().parent().parent().children()
                         .attr('data-original-title');