Sometimes tests are made when you start to work with Metadata discovery and KT, then data is replicated creating systems that are purely rubish. Other times, databases are not refined and tables are changed/removed etc, then the System fields version replicated keeps the picture what is not reflecting the reality of the metada in the source system. If we had that option these elements (systems and/or fields) could be cleaned them up to show a tidier view of the KT tenant.
The ability to delete System assets is now included in the SKP. You can tidy up your catalog from systems that you no longer need via the ... menu on the System details page as you can for all other assets.