api: Remove asset schema migration compatibility code

[victorges]

What does this pull request do? Explain your changes. (required)

This removes a compatibility code that I created for migrating assets to the new
schema a month ago on #1178.

With this, we delete 255 of tech debt to keep the code simpler. All assets have already been migrated
to the new format through the /migrate-status API, both in staging and in prod. This is given by this
response on the /migrate-status API on both of them:

{
    "count": 0,
    "nextCursor": null,
    "updated": []
}

Specific updates (required)

• Remove /migrate-status API
• Remove custom DB table with migration tricks
• Remove migration tests

-

• How did you test each of these updates (required)
  yarn test

Does this pull request close any open issues?
Kills unticketed tech debt.

Screenshots (optional):

Checklist:

• I have read the CONTRIBUTING document.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added removed tests to cover my changes.

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Name	Status	Preview	Updated
livepeer-studio	✅ Ready (Inspect)	Visit Preview	Sep 7, 2022 at 8:36PM (UTC)

[gioelecerati]

LGTM!