comments: remove unused property

• Removes an unused property on the CmtSUBSCRIPTION model, that also caused an initialization error due to the user of current_user proxy outside of the Flask application context.

Co-authored-by: Jiri Kuncar <jiri.kuncar@cern.ch>
Signed-off-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>