[kerning] defcon/robofab compat
[kerning] defcon/robofab compat

Actually, RoboFab kerning objects don't have a __delitem__ method, so
we need to try both delete() and del to be cross-compatible.

googlefonts/ufo2ft,jamesgk/ufo2ft,moyogo/ufo2ft,googlei18n/ufo2ft,jamesgk/ufo2fdk