drm/i915: Drop debugfs/i915_next_seqno

Having just gutted the implementation as there is no global seqno
tracking, remove the vestigal write-only stub for debugfs/i915_next_seqno.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181228140736.32606-3-chris@chris-wilson.co.uk