MDL-39412 question behaviours: hide manual graded option

The ability to set all your quiz questions to require manual grading is
an interesting possibility, but practically almost useless.

If you set that accidentally, then you are badly stuck. There is no way
to fix it after the students have answered the quiz.

Therefore, we should set the config option to hide that option from the
UI. We do this for all Moodle sites as part of the upgrade, not just for
new installs.

If any admin wants to re-enable this, they can later.