Remove Stephen Casner as the PDP11 maintainer

[Nick Clifton]

Hi Guys,

   I have sad news to impart.  Steven Casner, who has been acting as the
   PDP11 maintainer for many years now, past away last summer.

   I am removing his name and email address from the maintainers list.
   At the moment we do not have a PDP11 maintainer, although Paul Koning
   has offered to stand in if there is a real need.  But for now I will
   be acting as the maintainer in the hopes that only occasional patches
   will need reviewing.

Cheers
   Nick