Remove observer from mLoadQueue when we request to remove observer

Remove observer from mLoadQueue when we request to remove observer

That might be happen during ResourceReady signal observing

- Add
- Remove

If than, the added request added in mLoadQueue 2 times.
and Remove request try to remove observe there (but will be failed actually.)

So, the visual get LoadCompleted signal twice.

It might have some logical problem in AnimatedImageVisual.
So let we keep ensure that 1-observer can only emit only 1-time.

Change-Id: I3eee087118cf62761709b8416fe09cf52094d156
Signed-off-by: Eunki, Hong <eunkiki.hong@samsung.com>