Delete the mbuf macros that were used for the Mac OS/X port.

Delete the mbuf macros that were used for the Mac OS/X port.

When the code was ported to Mac OS/X, mbuf handling functions were
converted to using the Mac OS/X accessor functions. For FreeBSD, they
are a simple set of macros in sys/fs/nfs/nfskpiport.h.
Since r359757, r359780, r359785, r359810, r359811 have removed all uses
of these macros, this patch deleted the macros from the .h files.

My eventual goal is deleting nfskpiport.h, but that will take some more
editting to replace uses of the remaining macros.