When you are uninstalling Exchange 2007 Edge server, First you have to remove the edge subscription file.Because then only you can do the un installation.

Firstly remove the edge subscription from Hub Transport Server and restart the Active directory topology servcie after that go to the edge server take the Exchange Management shell type “Remove-EdgeSubscription

This command remove the edge Subscription file from the active directory.
After successful deletion you can start the uninstallation.