MDL-61718 admin: Make sure city and country columns are not duplicated

* Since City/town and Country have been added in $CFG->showuseridentity
since 3.4, we need to exclude the required city and country columns
when fetching the extra columns using get_extra_user_fields().

admin/user.php

@@ -160,10 +160,13 @@
 
     // Carry on with the user listing
     $context = context_system::instance();
-    $extracolumns = get_extra_user_fields($context);
+    // These columns are always shown in the users list.
+    $requiredcolumns = array('city', 'country', 'lastaccess');
+    // Extra columns containing the extra user fields, excluding the required columns (city and country, to be specific).
+    $extracolumns = get_extra_user_fields($context, $requiredcolumns);
     // Get all user name fields as an array.
     $allusernamefields = get_all_user_name_fields(false, null, null, null, true);
-    $columns = array_merge($allusernamefields, $extracolumns, array('city', 'country', 'lastaccess'));
+    $columns = array_merge($allusernamefields, $extracolumns, $requiredcolumns);
 
     foreach ($columns as $column) {
         $string[$column] = get_user_field_name($column);