D14130: stand/fdt: Rip out FDT VA tracking

Whether we should be overwriting the loaded FDT module with the 'fixed up'
version or not was questionable when this was added, and now that overlays
are possible this is downright wrong.

Overlays can increase the size of the blob, so writing it back to the
original VA will generally write past the end of the block and start
clobbering other things in memory.

Rip it out- it was questionable to begin with, it's doing bad things now,
and it serves no purpose since the modified blob will be copied into place
rather than relying on this to reflect the changes.

Reviewed by: gonzo
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D14130