Revert "i18n lon-capa problem explanation title"

This reverts commit 3e0342cb.

common/lib/xmodule/xmodule/js/src/problem/edit.coffee

@@ -201,8 +201,6 @@ class @MarkdownEditingDescriptor extends XModule.Descriptor
 #
   @markdownToXml: (markdown)->
     toXml = `function (markdown) {
-      // Translators: This is the title of the text explaining a problem
-      var explanation_title = gettext("Explanation");
       var xml = markdown,
           i, splits, scriptFlag;
 
@@ -350,9 +348,8 @@ class @MarkdownEditingDescriptor extends XModule.Descriptor
       });
 
       // replace explanations
-
       xml = xml.replace(/\[explanation\]\n?([^\]]*)\[\/?explanation\]/gmi, function(match, p1) {
-          var selectString = '<solution>\n<div class="detailed-solution">\n' + explanation_title + '\n\n' + p1 + '\n</div>\n</solution>';
+          var selectString = '<solution>\n<div class="detailed-solution">\nExplanation\n\n' + p1 + '\n</div>\n</solution>';
 
           return selectString;
       });