This is more of a SharePoint or regular PowerShell question than an Azure AD question, and these products are supported on Microsoft Answers. I would suggest checking there for this. https://answers.microsoft.com/en-us
I would also check out this similar thread here, which says:
To remove a workflow association from a list or document library
- Verify that you have the following administrative credentials:
You must be a member of the Site Owners group on the SharePoint site that you are configuring.
- Browse to the list or library from which you want to remove a workflow.
- Do one of the following:
For a list, on the List Tools tab, click List.
- In the Settings group, click List Settings.
For a library, on the Library Tools tab, click Library.
In the Settings group, click Library Settings.
On the List Settings or Library Settings page, in the Permissions and Management section, click Workflow Settings.
- On the Workflow Settings page, click Remove a workflow.
Note - The Remove a workflow link is not displayed if no workflows are associated with the list or library.
- On the Remove Workflows page, find the workflow that you want to remove and choose from the following options:
Select No New Instances to prevent new instances of this workflow from running but still allow for running instances to finish.
After all running workflows are finished, you can return to this page to completely remove the workflow association.
Select Remove to prevent new instances of this workflow from running and remove all instances of this workflow. This option removes instances that are already running.
- Click OK.