I don't have a partitioned drive with which to double-check this, but in Disk Utility, when I select the "Partition" option, there is a +/- below the drive image. Select "-" to delete the partition? I would think you will get a warning that you will lose data, but that's OK in your situation. Then erase the single remaining partition.

On a Mac since 1984.
Currently: 24" M1 iMac, M2 Pro Mac mini with 27" BenQ monitor, M2 Macbook Air, MacOS 14.x; iPhones, iPods (yes, still) and iPads.