[nmh-workers] Removing message/partial support

[Ken Hornstein]

As discussed here:

        https://lists.gnu.org/archive/html/nmh-workers/2019-02/msg00035.html

Currently our message/partial support does not work.  I believe fixing
it is straightforward, but given the links that David posted in that
thread and my own research, almost no other MUAs support it and it is
probably blocked by most mail providers due to concerns about viruses
and spam.  Can anyone think of a reason to keep it?  I suggest removing
all support for it, on both sender and receiver sides.

--Ken