How to supply an unsigned Android build?

A client is requesting that we supply them an UNsigned version of an Android build, that they intend to sign themselves.

In Build Settings I selected "Create a new Keystore", and then under Key I selected "Unsigned", but the client came back and said that it was not unsigned. I tried once more, this time without selecting or creating a keystore at all, but they are still saying it's no good and that they need a version "without a developer certificate applied".

Someone has suggested that I should check "Development Build" in the Build Settings, but surely this would run slightly slower than a release build (as it contains hooks for profiling and debugging etc)?

Or perhaps do I need to export as "Google Android Project" and supply the whole thing to the client so they can build themselves?

Thanks!