Revert delegate setting related commits

This reverts the following commits:
- efc4f9948db0a89435627bde32d2232efc04d020 (modify delegate setting)
- bca811fc9618236c937d3945834d81201a7080ed (comment on the modification
that was made)

As it turns out, this is:
1. A typo (it should check if `self != nil`)
2. Not setting `extVars.collectionViewDelegate` when `== self` is not the right solution, since it blocks calls to `PSTCollectionView`'s functional implementation of UIScrollViewDelegate methods (such as `scrollViewDidEndDragging:willDecelerate:`)

As I said in #251, I'm going to try to use an instance of `HTDelegateProxy` and remove `PSTCollectionView`'s UIScrollViewDelegate method forwarding.  Perhaps Apple is using something similar to `HTDelegateProxy` to forward invocations, while avoiding the callback loop.