Fixing bugs in the objectionable items refactor

Bug 1298646: No point in adding columns to the interaction_forum_post table
just to immediately drop them again.

Also, there were some lingering "accesstype IS NULL" checks in the elasticsearch
access queries, which should have been removed along with the "accesstype != objectionable"
that they were previously parenthetical with.

And, the artefact_access table doesn't need to accept "objectionable" as 
an accesstype enum value anymore.

Change-Id: I961f15e98eb8aab95e4e5b16af223eb61588f671
Signed-off-by: Yuliya Bozhko <yuliya.bozhko@totaralms.com>