Delete unused function newnfs_trimleading.

The NFS function called newnfs_trimleading() has not been used by the
code in long time. To give you a clue, it still had a K&R style function
declaration.
Delete it, since it is just cruft, as a part of the NFS mbuf handling
cleanup in preparation for adding ext_pgs mbuf support.
The ext_pgs mbuf support for the build/send side is needed by
nfs-over-tls.