gistfile1.txt

It's possible to unmap humanoid bones in a VRM, but it's a bit tricky.
You have to find the VRM Humanoid Description component and double click
the AvatarDescription on there. Then you should see a list of all the
mapped humanoid bones in the inspector by alt+clicking the triangle in
front of human. Look for the bone you want to unmap, then scroll all the
way to the bottom and take a look at the very last bone. Copy all the
entries on the last bone into the slot for the bone you want to unmap,
including the name. Then go back to the top where the number of items in
the bone list is given. Reduce it by one and it should cut off the last
bone in the list, so now the information is only on the one you overwrote.
If you reexport the VRM now, the bone you overwrote with the one from the
bottom should be unmapped.