Would it be save to "drop" these? It scares me a little to make changes to my database outside the CMS
As with any db operation best to make a backup (and make sure that you can restore that backup) before you start.
I was worried about left over fields too - if I correct a spelling mistake in the db, dev build I have two very similarly named fields... but not any more thanks to http://github.com/smindel/silverstripe-dbplumber it has an "artefacts" function that creates sql (and runs it) that removes these obsolete fields and tables.