Contributed By: Cliff Hobbs [MVP ConfigMgr]
There's isn't an easy, complete, uninstall process for MDT 2008. So if you want to delete the database you need to:

1. Close the Deployment Workbench if it is open.

2. Load SQL Server Management Studio (assuming you are using SQL 2005) and connect to the database.

3. Expand the Databases node in the console.

4. Right-click on the MDT database and from the context menu select Delete.

5. Click OK on the Delete Object screen.

6. Close SQL Server Management Studio.

7. Navigate to <drive_where_MDT_installed>:\<MDT_Distribution_share_name>\Control

8. Delete the Databases.xml file.