R1:52c4715bbcfb

No-op the Conpherence thumbnail resizing migration

Summary:
Fixes T12628. After later changes to `PhabricatorFile`, this migration no longer runs if you upgrade through it to a recent `HEAD` while your data has some room images.

Since this isn't critical and has been available for ~6 months, I just nuked it as a first pass. I can find a more careful approach which lets us continue to run this migration instead if you're hesitant to skip this step, although it may be a little involved.

In 95% of cases we…