Delete a Database Diagram (Visual Database Tools)
Applies to: 
SQL Server
Azure SQL Database
Azure SQL Managed Instance
Analytics Platform System (PDW)
When you no longer need a database diagram, you can delete it.
To delete a database diagram
In Object Explorer, expand the Database Diagrams folder.
Right-click the database diagram that you want to delete.
Choose Delete on the shortcut menu.
A message prompts you to confirm the deletion. Choose Yes.
The database diagram is deleted from the database. When you delete a database diagram, the tables in the diagram are not deleted.