In some cases, removing the associated assessment will delete the column. Other times it will enable you to delete the column in the Grade Center. In the case of a graded discussion forum, it is important to edit the forum and disable the grading before deleting the forum, or the Grade Center column will be left and you will be unable to delete it.
I deleted a graded discussion forum, but the column is still in the Grade Center and there is no option to delete it.
When a discussion forum is created as a graded forum, a column is automatically created in the Grade Center. If the discussion forum is then deleted, the column remains in the Grade Center and cannot be deleted. This issue can also occur during a Course Copy, if Grade Center Settings are copied but the Discussion Board is not (and graded discussion forums exist).
There are two workarounds: Faculty can hide the column from the student view and then hide the column from the instructor view OR faculty can use Bulk Delete to remove all Grade Center columns and settings. WARNING: using Bulk Delete to delete the Grade Center can permanently delete student grades and any Assignment or Test submissions that have been made.