MDL-79705 reportbuilder: remove checkbox field callback handling.

We no longer need special handling of this field type, because it
now returns language strings rather than HTML markup for a checkbox
element on display.

reportbuilder/classes/local/helpers/user_profile_fields.php

@@ -180,9 +180,8 @@ class user_profile_fields {
                 ->set_type($columntype)
                 ->set_is_sortable($columntype !== column::TYPE_LONGTEXT)
                 ->add_callback(static function($value, stdClass $row, profile_field_base $field): string {
-                    // Special handling of checkboxes, we want to display their boolean state rather than the input element itself.
-                    if (is_a($field, 'profile_field_checkbox')) {
-                        return format::boolean_as_text($value);
+                    if ($value === null) {
+                        return '';
                     }
 
                     $field->data = $value;

reportbuilder/tests/local/helpers/user_profile_fields_test.php

@@ -240,7 +240,7 @@ class user_profile_fields_test extends core_reportbuilder_testcase {
             [
                 'c0_firstname' => 'Admin',
                 'c1_data' => '',
-                'c2_data' => 'Not set',
+                'c2_data' => '',
                 'c3_data' => '',
                 'c4_data' => '',
                 'c5_data' => '',