remove legacy text export, fixes #68

controllers/sheets.php

exercises/Exercise.php

exercises/cloze_exercise.php

exercises/lt_exercise.php

exercises/mc_exercise.php

exercises/mco_exercise.php

exercises/me_exercise.php

exercises/pl_exercise.php

exercises/rh_exercise.php

exercises/sc_exercise.php

exercises/tb_exercise.php

lib/vips_common.inc.php

views/sheets/export_assignment_text.php