Remove unnecessary ContentPermissionSet object and database table

There really is no good reason to have content permission sets in Confluence. Content should related directly to ContentPermission. This will simplify our permission checking, and would have gone some of the way in preventing weird bugs like CONF-19914. I've spoken to Charles about this already - in fact he brought up the idea before I could even get to it.

I originally planned to do this as part of CONF-22123 but didn't have enough time so I had to raise it as a separate task.