Debug warning when restoring an activity when multiple question_categories are at the same context level

A minor bug resulting in debug output to the page, when in developer mode.

To replicate it:

1. Make sure developer mode is on (DEBUG).
2. Create a new course under the miscellaneous course category
3. From within the question bank in the course, create 2 new question categories at the course-category level. (Course admin > Question bank > Categories)
   • Select new category
   • make sure the parent is 'Top' under the 'Miscellaneous' course category.
4. you should now see three question categories in the section, 'Question categories for category: Miscellaneous':
   • Default for miscellaneous
   • First question category you created
   • Second question category you created
5. create a single question under any one of the course-category level question categories
6. create a quiz, making sure to use that question you just created
7. Now, backup the course (you can use 'Jump to final step')
8. Restore the course into a new course, under the miscellaneous course category using all the default settings.
9. You will see something like this:

   Did you remember to make the first column something unique in your call to get_records? Duplicate value '3' found in column 'contextid'.

   line 784 of /lib/dml/pgsql_native_moodle_database.php: call to debugging()

   line 803 of /backup/util/dbops/restore_dbops.class.php: call to pgsql_native_moodle_database->get_records_sql()

   line 602 of /backup/util/dbops/restore_dbops.class.php: call to restore_dbops::restore_find_best_target_context()

   line 519 of /backup/util/dbops/restore_dbops.class.php: call to restore_dbops::prechek_precheck_qbanks_by_level()

   line 178 of /backup/util/helper/restore_prechecks_helper.class.php: call to restore_dbops::precheck_categories_and_questions()

   line 378 of /backup/controller/restore_controller.class.php: call to restore_prechecks_helper::execute_prechecks()

   line 990 of /backup/util/ui/restore_ui_stage.class.php: call to restore_controller->execute_precheck()

   line 372 of /backup/util/ui/restore_ui.class.php: call to restore_ui_stage_process->process()

   line 99 of /backup/restore.php: call to restore_ui->requires_substage()

The warning is generated by a call to get_records_sql in restore_find_best_target_context(), which doesn't ask for a unique column correctly.