librbd: remove special case for starting AioCompletion ops

librbd: remove special case for starting AioCompletion ops

All ops can be immediately started now that flush ops won't
accidentally block themselves.

Signed-off-by: Jason Dillaman <dillaman@redhat.com>
(cherry picked from commit b5fc7ecaf7a3741d227ab5a9108392813dd03495)

Conflicts:
	src/librbd/api/DiffIterate.cc: dropped changes
	src/librbd/image/CloseRequest.cc: dropped changes
	src/librbd/image/RefreshRequest.cc: dropped changes
	src/librbd/io/AioCompletion.cc: assert/ceph_assert conflicts
	src/librbd/io/ImageRequest.cc:: assert/ceph_assert conflicts
	src/librbd/io/ImageRequestWQ.cc: trivial resolution
	src/librbd/operation/ResizeRequest.cc: dropped changes
	src/test/librbd/journal/test_Replay.cc: dropped changes
	src/test/rbd_mirror/test_ImageSync.cc: dropped changes