drm/i915: Remove debugfs/i915_seqno_info

The per-engine seqno info is now available from
debugfs/i915_engine_info obsoleting debugfs/i915_seqno_info, so remove it.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171209104418.4223-1-chris@chris-wilson.co.uk