To Delete a Main Task

In the Customize dialog ( File > Settings > Configuration > Customize ) on the Tools tab, expand the Main Tasks tab.
Expand the User Tasks section and expand the open DGN library.
Select the Main task that you want to delete.
Do one of the following:

Click the Delete icon.

or
Right-click the Main task and select Delete from the pop-up menu.

or
Press the <Delete> key.