Remove gradable-nbexport
Remove gradable-nbexport

I archived gradable-nbexport, and it is not needed
anymore https://github.com/berkeley-dsep-infra/datahub/issues/930

ryanlovett/datahub,ryanlovett/datahub,ryanlovett/datahub,berkeley-dsep-infra/datahub,berkeley-dsep-infra/datahub,berkeley-dsep-infra/datahub