According to a Google customer service rep I contacted:
The Deactivate button is provided in your advanced mode when you want to support multiple APKs but when your new update supports the same number of devices as your previous APKs, the system won't allow such update. So it is required for you to deactivate the older version in order to upload your new APK.
The Deactivate
button is only useful for situations where you have multiple active APKs.
Furthermore, discussing with them, there is apparently no functionality at this time that allows for you to remove APKs from the Production tab once they've been uploaded to that tab, AND there's apparently no way to enable Beta testing if your app is unpublished, so at this point it seems there's no way to undo what you've done if you accidentally upload an APK to the Production tab by mistake. The workaround proposed by the representative was to change the package name of the app, change the keystore used to sign, and create an entirely new application.
Further discussion with the representative indicated that the "error" is that you can't have a Production tab without an APK if there was one there already, but it sounds like there's a bug in that it doesn't actually report that that's the cause.