mips: Remove mips

* As per the ML discussions. Bumps MIPS to tier 3.
* We've reached a unanimous descision that MIPS doesn't
  target any real / valid hardware Haiku wants to pursue
  at the moment.  In the event that anyone wants to pursue
  MIPS, feel free to fork Haiku into your own repository
  (and we'll even link to it on the website ports page)
* If someone develops a viable plan for MIPS (and gets the
  port working, it can be readded at a later date)