Remove the MBR_init() "#ifdef defined(__macppc__) ||

Remove the MBR_init() "#ifdef defined(__macppc__) ||
defined(__mips__)" chunk that rounded the start of the default
OpenBSD partition to a cylinder boundary. The value has been
immediately re-rounded to a power of 2 block since r1.25 in 2009.

Eliminates wasted space when no /usr/mdec/mbr partition
0 information is available.

'-b' becomes available to architectures other than amd64 and
i386, taking precedence over /usr/mdec/mbr partition 0
information. The latter being present only in macppc and loongson
/usr/mdec/mbr files.