Asynchronous zone load events have no way of getting canceled

Asynchronous zone load events have no way of getting canceled

Code handling cancellation of asynchronous zone load events was likely
copied over from other functions when asynchronous zone loading was
first implemented in commit 8a2ab2b9203.  However, unlike those other
functions, asynchronous zone loading events currently have no way of
getting canceled once they get posted, which means the aforementioned
code is effectively dead.  Remove it to prevent confusion.