nfscl: Clear out a lot of cruft related to B_DIRECT

There is only one place in the unpatched sources where B_DIRECT is
set in the NFS client and this code is never executed. As such, this patch
removes this code that is never executed, since B_DIRECT should never
be set.

During a IETF testing event this week, I saw a crash in ncl_doio_directwrite(),
but this function is only called if B_DIRECT is set.
I cannot explain how ncl_doio_directwrite() got called, but once this patch
was applied to the sources, the crash did not recur. This is not surprising,
since this patch deleted the function.

Reviewed by: kib, markj
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D44980