To create the advanced list to see what assessments grades are tied to assessment grade plans, you will go to:
  1. Navigate to Lists > Manage Lists
  2. Click the Manage basic and advanced lists block under List Actions
  3. Click the List Templates tab
  4. Select Category: Grading
  5. View/Copy the list "Current Year Academic Grade Detail"

On the Selected Objects tab:
  1. Select Assessment Grade and click Assessment Grade Plan Grade
  2. Still under Assessment Grade, click Assessment Grade
User-added image
On the Display Fields tab:
  1. Click Select Fields
  2. Expand User Base > Course Enrollment > Enrollment Grade > Assessment Grade Plan Grade
  3. Mark Grade Description
  4. Select
On the Filters tab, you can select to choose if you want to filter by any particular grade plan or school year. When Clicking Preview in the bottom right, you'll see what grades are included in that particular grade plan.

You can then remove these grades and should then be able to delete the assessment grade plan once all those grades are removed from it.