NOTES: Remove paragraph about slow-to-prepare messages

NOTES: Remove paragraph about slow-to-prepare messages

NOTES contained this paragraph:

  Some messages may take a long time to prepare (software modexp on slow
  machines); this is a "please wait" message to indicate that a message
  is in preparation.

This paragraph was immediately after the description of the msg0(msg9)
packet - ie, the ordinary data packet.  It is therefore at the very
least misplaced.

In the git history this paragraph was introduced in "Import release
0.1.14" (4f5e39ec).  However, the diff for that commit (ie the diff
between 0.1.13 and 0.1.4) does not show any code which might
correspond to this comment.

There is not currently any code in site.c which responds to a message
of this kind.  I have had a quick look for code which sends such a
message and failed to find any.

So I think this paragraph represents a never-implemented intent, and
should be removed.  It certainly predates the "hacky parallelism" by a
long way.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>