Revert "Launches switching ASNetworkImageNode callbacks to global queue."

[Adlai-Holler]

Reverts #1369

This is a pretty huge API change! I'm uncomfortable doing it so quietly. Some users undoubtedly do main-only things in these callbacks. For example, they might do if (self.hidden) { self.hidden = NO; } which would be accessing a bridged property off-main and boom.

I propose instead adding an opt-in class flag to trigger the feature. It's still an optional feature, it's just not an experimental one anymore. Like so:

// ASImageNode.h
AS_EXTERN void ASImageNodeEnableBackgroundCallbacks(void);

[wsdwsd0829]

BTW, we had ASNetworkImageNode.useMainThreadDelegateCallbacks to optionally diable setting to NO. to control image delegate call backs.

[garrettmoon]

I don't think this is an API change, it's used entirely internally as far as I can tell?

[Adlai-Holler]

Oh, you're right, my bad! let's go forward with this.