Good point. Do remove the eMMC.
If you've never done this, it isn't obvious, but you just pull it straight up to remove it.
I should mention something else that isn't obvious: all the boot code (including U-boot) exists on the boot device.
If you have boot code on the SPI F-ROM, that runs at startup.
Else, if you have boot code on the eMMC, and it's enabled, that runs at startup.
Else if you have boot code on the micro SD card, that runs.
So unless there's something on the SPI F-ROM (very unlikely), as long as the eMMC can run, a corrupt U-Boot on there will prevent you booting any other way.
If you've never done this, it isn't obvious, but you just pull it straight up to remove it.
I should mention something else that isn't obvious: all the boot code (including U-boot) exists on the boot device.
If you have boot code on the SPI F-ROM, that runs at startup.
Else, if you have boot code on the eMMC, and it's enabled, that runs at startup.
Else if you have boot code on the micro SD card, that runs.
So unless there's something on the SPI F-ROM (very unlikely), as long as the eMMC can run, a corrupt U-Boot on there will prevent you booting any other way.