Bug 13006: suggestions.mailoverseeing cans be removed

Bug 13006: suggestions.mailoverseeing cans be removed

The DB field suggestions.mailoverseeing does not seem to be in used.
It cans be removed.

Actually it has never been in used.

Test plan:
git grep mailoverseeing
should not return relevant results.

Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>
Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Passes tests, updates kohastructure and includes a database update.
Also made some tests in the suggestions module.