With the move to Nodepool in Zuul we're no longer providing ARM64 Bionic
nodes. Drop the mirror content for Bionic on this architecture.
Note that this won't actually remove the content. It will simply stop
spending time trying to update the content. We will need to take manual
action to clear th old data out.
Change-Id: I1f901dde0db987ea96d50431f1d2e0e0f941d5b5
f4e7b20690