drm/i915/gt: Remove errant assertion in __intel_context_do_pin

This assertion was removed in commit b412c63f ("drm/i915/gt: Report
context-is-closed prior to pinning"), but accidentally restored by a
cherry-pick into drm-next and now has percolated back to
drm-intel-next-queued.

Fixes: 2e46a2a0 ("drm/i915: Use explicit flag to mark unreachable intel_context")
Fixes: 2b703bbd ("Merge drm/drm-next into drm-intel-next-queued")
References: b412c63f ("drm/i915/gt: Report context-is-closed prior to pinning")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200520073048.2394034-1-chris@chris-wilson.co.uk