Our CI jobs generally don't make use of source packages, and because
pulling them requires a separate line in sources.list anyway any
jobs which want them can add an official mirror source easily.
Remove from the configured architectures list in ubuntu-ports
initially, since ARM jobs are even less likely to be impacted by
this due to their scant number and limited frequency. This will
allow us to evaluate the fallout and determine whether manual
cleanup is needed afterward.
Change-Id: Ib647b6733529d965fa9caa92ad4d123639510083