server: Remove deprecated diagnostic feature

This commit removes the deprecated diagnostic feature from the
server. The feature has been deprecated since November 2018 and it was
essentially unused at the time so it should be safe to
remove. Removing the diagnostic support from the server saves having
to perform an extra policy evaluation in the server.

Once the buffer is removed from the runtime.Params struct the related
issue can be closed (there is still one known user of that so it has
been left intact for backwards compatibility.)

Ref #1052

Signed-off-by: Torin Sandall <torinsandall@gmail.com>