How do I uninstall old SDKs?
The installer for Xcode 4.3 had an option for removing older versions of Xcode which removed /Developer/.

You can probably just delete the folder completely. It was possible to delete single developer folders even before Xcode 4.3. From an answer by Chris Espinosa on Stack Overflow:


  If you have multiple Developer folders, you can drag any of them to the trash without damaging the others. Developer folders are completely standalone.