Update hook_uninstall, remove variable_delete() on variables those are already converted to question type entity

Follow previous PR

- long_answer_default_max_score
- multichoice_def_num_of_alts
- multichoice_def_scoring
- quiz_ddlines_canvas_height
- quiz_ddlines_feedback_correct
- quiz_ddlines_feedback_wrong
- quiz_ddlines_hotspot_radius
- quiz_matching_form_size
- quiz_matching_shuffle_options
- scale_max_num_of_alts
- short_answer_default_max_score