First, only running software consumes memory/CPU/etc, non-running sofware puts no load on your machine. If you don't start the Promise Utility it will not consume any resources, if you don't attach a Pegasus, the driver will not be loaded. If that is your primary concern you need do nothing else.
That being said, to delete the Promise Utility just open finder, go to Applications, find the promise Utility and drag it into the trash. You'll probably need the admin password for this.
Deleting the driver is similar, the bundle name is probably PromiseSTEX.kext (kext is for Kernel Extenstion, aka the driver). In my High Sierra install I find the bundle in /Library/Extensions, in older macOS versions it might be in /System/Library/Extensions. Just open the filer, go to the appropriate folder, find the kext and drag it into the trash. You will probably need an admin password for this as well.