drm/i915/guc: Remove obsolete comments and remove unused variable

Originally removed in:
c1adab97 ("drm/i915/guc: Remove failed doorbell stat from debugfs")
f1448a62 ("drm/i915/guc: Remove last submission result from debugfs")

Were accidentally restored in:
925344cc

 ("BackMerge tag 'v4.12-rc5' into drm-next")

We can also remove unused variable and replace it with a WARN.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: Michał Winiarski <michal.winiarski@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20170914083216.10192-1-michal.winiarski@intel.com
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>