I am able to delete the non-associative dimensions (converted legacy dimensions).
Enter the Dimension Environment, Enable "Delete Dimensions" from the Right Mouse Button popup then Select the dimension to delete.
I am still running the SPB 16.5 base install and have not loaded the Hotfix yet but maybe you have and it is now broke.
Give it another try, it took a little time to get use to the whole Dimension Environment in 16.5
Hope this helps,
Plexus Engineering Solutions