Remove suspected merge conflict in darwin_threading.c.
Remove suspected merge conflict in darwin_threading.c.

Fixes: #4374

iree-org/iree,iree-org/iree,google/iree,google/iree,iree-org/iree,google/iree,iree-org/iree,iree-org/iree,google/iree,google/iree,google/iree,iree-org/iree,google/iree,iree-org/iree