Filter spurious concepts in Learner Model

Due to some shenanigans, the SPARQL interface might currently be including some terms like term 4 or term 57 that are, of course, not real concepts and are only unnecessarily cluttering the user interface.

The suggestion of @do45qahi was to delete them via workaround, the upcoming new backend will not have this problem. I think for our purposes we can just filter out interaction events that contain only these and delete them if they're not the only ones.