Garbage collect m_copymdata(), an mbuf utility routine introduced

Garbage collect m_copymdata(), an mbuf utility routine introduced
in FreeBSD 7 that has not been used since.  It contains a number
of unresolved bugs including an inverted bcopy() and incorrect
handling of read-only mbufs using internal storage.  Removing this
unused code is substantially essier than fixing it in order to
update it to the coming mbuf world order -- but it can always be
restored from revision history if it turns out to prove useful for
future work.

Pointed out by:	jmallett
Sponsored by:	EMC / Isilon Storage Division