messaging vs. message +: Storage full

[Android Central Question]

2015 Motorola Droid Turbo. I've been getting notifications that internal storage is full. I've deleted some apps and data, but more recently I've been notified that Messaging storage is full, and I won't get texts. I've never used Messaging, as I use Message + instead. I still get texts on that, though I get an annoying "text rejected" notification from Messaging. As I was reviewing my storage data, I discovered that all of my text messages were stored in both apps. How can I delete the data from Messaging? It looks like I can't uninstall it from the phone.