drm/i915/guc: Remove racey GEM_BUG_ON

A full GT reset can race with the last context put resulting in the
context ref count being zero but the destroyed bit not yet being set.
Remove GEM_BUG_ON in scrub_guc_desc_for_outstanding_g2h that asserts the
destroyed bit must be set in ref count is zero.

Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211214170500.28569-4-matthew.brost@intel.com