url_preview media cache is not expired when background database migrations are running

[richvdh]

Normally, any media downloaded as part of a url preview is deleted after a day. However, while a background database migration is running, the process is stopped. If there is a long-running database migration, this can therefore cause the media store to fill up with old preview files.

This logic was added in #2697 to make sure that we didn't try to run the expiry without an index on local_media_repository.created_ts; the original logic that needs that index was added in #2478 (in get_url_cache_media_before, as amended by 93247a4), and is still present.

It looks like too broad a constraint: instead, we should use has_completed_background_update to check if the specific background update we care about has completed. However, given you'd have to be upgrading from before Synapse v1.0.0 (which created the v54 schema snapshot) to not have this index in place... we should probably just drop the check altogether.

[richvdh]

TL;DR: let's just remove this check.

[clokep]

any media downloaded as part of a url preview is deleted after a day

Just to avoid confusion -- they're deleted after two days. I don't think this changes any of the rest of the issue though. Thanks for researching!