23507:correctly handle non-netable

Drop the column instead of updating it. The update is destructive (there's no way to reconstruct the old version of the data from the new version) so there's little point in keeping it. There will be some support fallout since customized reports will start to fail but it's probably better than the alternative - customized reports continue to run and we get questions 2 years from now about why the NNQOH is always 0.

@piercetyler might have a different opinion.