Revert "Register new snapshots"

This reverts commit 9b443289cf32cbcff16768614340f0c844675340.
auto merge of #19645 : alexcrichton/rust/old-snap, r=brson

The most recent snapshot was produced on OSX 10.8, but this segfaults on OSX
10.7 so we need to roll back one snapshot so we can start bootstrapping on 10.7
systems again.

cc #19643