nil out tableView delegate on dealloc

If tableView is scrolling as this VC is being dealloc'd
it continues to send messages (scrollViewDidScroll:) to its delegate.
This is fine if the delegate will outlive tableView (e.g. this VC would.)
However, if the delegate is an instance that may be dealloc'd
before the tableView
(i.e. _scrollProxy may be dealloc'd prior to tableView being dealloc'd)
the tableView will send messages to its delegate,
which is defined with an "assign" (i.e. unsafe_unretained) property.
This is a msgSend to non-nil'ed, invalid memory leading to a crash.
If or when UIScrollView's delegate is referred to with "weak" rather
than "assign", this can and should be removed.