drm/i915: Remove ironlake rc6 support

Apparently, this has never worked reliably and is currently disabled. Also, the
gains are not particularly impressive. Thus rather than try to keep unused code
from decaying and having to update it for other driver changes, it was decided
to simply remove it.

For: VIZ-5115
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>