drm/i915/gt: Drop a defunct timeline assertion

intel_timeline_enter() has been decoupled from intel_timeline_pin() and
both enter/exit & pin/unpin are allowed [read expected] to run
concurrently with one another. The assertion that they had better not is
stale.

Closes: https://gitlab.freedesktop.org/drm/intel/issues/940


References: a6edbca7 ("drm/i915/gt: Close race between engine_park and intel_gt_retire_requests")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200107143826.3298401-1-chris@chris-wilson.co.uk