Were you able to connect the UART cables and open a serial console ?
You'll need to interrupt U-Boot before it loads anything, and then type
ums 0 mmc 1 in U-Boot prompt, through the serial console.
Once done, the whole eMMC will be accessible like an external USB drive, that you can format using fdisk or gparted.
Be CAREFUL with gparted. Be sure to apply operations on the USB drive.
Also, the first partition must start after the first 32 MB, just be sure to not overwrite anything related to U-Boot.