Migrate away ci_builds.trace

I did check on GitLab.com how much data is stored in ci_builds.trace column. This was used when GitLab CI was still separate product. We then moved to storing traces on disk. According to my queries, it is:

$ Ci::Build.where.not(trace: nil).count
=> 87636
$ Ci::Build.where.not(trace: nil).pluck('sum(octet_length(trace))')
=> [5608799010]

It is over 5.5GB, on average over 60k per build.

We should prepare Background Migration to move that to disk.

cc @grzesiek