Removing a solution platform from Configuration Manager

See the msdn:

http://msdn.microsoft.com/en-us/library/ms165408.aspx

Removing a Platform
If you realize that you have no need for a platform, you can remove it
using the Configuration Manager dialog box. This will remove all
solution and project settings that you configured for that combination
of configuration and target. To remove a platform

1. On the Build menu, click Configuration Manager.
2. In the Active solution platform box, select <Edit>. The Edit Solution Platforms dialog box opens.
3. Click the platform you want to remove, and click Remove.