clean up old 6.7 softraid migration code

clean up old 6.7 softraid migration code

ofwboot still passes an old/small .openbsd.bootdata size from before 6.7
when boothowto was added.

Report the exact size from now on such that a future diff can rectify
the corresponding check in autoconf.c:bootstrap().

All this was done to keep old/new bootloaders working with new/old kernels,
but 6.7 is long gone and we should all be running current code.

OK stsp