MDL-61165 core: Final deprecation and removal of legacy cron.

Following MDL-52846 this now finally deprecates and removes the
following classes:
- \core\task\legacy_plugin_cron_task.
- \mod_quiz\task\legacy_quiz_reports_cron
- \mod_quiz\task\legacy_quiz_accessrules_cron
- \mod_workshop\task\legacy_workshop_allocation_cron

Please, use the Task API instead:
https://moodledev.io/docs/apis/subsystems/task

This also removes the corresponding and specific to legacy cron strings
from mod_quiz and mod_workshop.

Following MDL-52846 this now finally deprecates and removes the
functions:
  - cron_execute_plugin_type()
  - cron_bc_hack_plugin_functions()

Please, use the Task API instead:
https://moodledev.io/docs/apis/subsystems/task

Signed-off-by: Daniel Ziegenberg <[email protected]>

lib/db/tasks.php

@@ -131,15 +131,6 @@ $tasks = array(
         'dayofweek' => '*',
         'month' => '*'
     ),
-    array(
-        'classname' => 'core\task\legacy_plugin_cron_task',
-        'blocking' => 0,
-        'minute' => '*',
-        'hour' => '*',
-        'day' => '*',
-        'dayofweek' => '*',
-        'month' => '*'
-    ),
     array(
         'classname' => 'core\task\grade_cron_task',
         'blocking' => 0,