I originally had a 2 sheet drawing, sheet 1 had a part detail (used a part file). Sheet 2 had an assembly procedure (used an assembly file). I decided to remove sheet 2, but I can't remove the assembly reference from drawing file to check it into vault. I tried the following, File>Open [References button] but I can't swap the assembly file for the existing part file. What are the options to remove/delete the reference?