right-klick on the soundpool and there you'll have the button to delete it.
Is it necessary to do this if one is merely "upgrading" to the commercial version by buying the commercial/professional licence?
Definitely not. Just to confirm (what you probably already knew to be true): The commercial licensing procedure is just about meeting legal requirements. Carrying out the purchase process for the pro license is all you need to do that. There's no need to actually download and install the files from Producer Planet or Catooh, if you already have the files installed from the non-commercial version of the soundpool. This is especially true in the current incarnation of Music Maker and the in-app store system, now that you already have the chance to get the files for the non-commercial soundpools in CD-quality WAV format. (This may have been a different story in the past, when Music Maker would only have come with lossy compressed OGG Vorbies copies of the bundled, non-commercial soundpools, and acquiring the commercial license from Catooh would have given you the high-quality WAV files later on. Some users may have been tempted to rebuild their entire song based on the WAV files to benefit from the potential advantage in audio quality....)