drm/i915: remove the i915_batchbuffer_info debugfs file

With the error_state facility in place, this has outlived it's
usefulness. It also oopses with the lates llc-reloc patches because
it directly access objects through the gtt without any checks.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>