drop i386 from default arches for now
    
    Summary:
    We've never quite set things up to run 32-bit tests. We should,
    at some point, but for now we're just wasting a bunch of time/
    bandwidth downloading them and never testing them.
    
    Test Plan:
    Schedule a run and make sure all the same tests are still run,
    but half as many images are downloaded...
    
    Reviewers: jskladan, garretraziel
    
    Reviewed By: garretraziel
    
    Subscribers: tflink
    
    Differential Revision: https://phab.qadevel.cloud.fedoraproject.org/D455