Define AOI is not necessary in Workflow Manager system. Your workflow jobs can exist without AOI. If you are looking at the sample workflow provided in the Workflow Manager database and update it based on your organization request, you can remove the Define AOI step from the workflow. You can also design your own workflow without the step.
The following link has the Workflow custom steps that we currently have. You can also explore other topics for related questions.
Workflow Manager custom steps—Help | ArcGIS Desktop
