chore: Remove old theming migration steps as they are only needed once

[juliusknorr]

Those migration steps were introduced with Nextcloud 26 to migrate config values but are no longer required on subsequent major upgrades. On instances with a large user base the MigrateUserConfig can take a large amount of time to iterate over users that already have been migrated (50 min on a 3 million user instance I've seen)

[juliusknorr]

/backport to stable28

[juliusknorr]

/backport to stable27

[backportbot]

The backport to stable27 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable27
git pull origin stable27

# Create the new backport branch
git checkout -b backport/43570/stable27

# Cherry pick the change from the commit sha1 of the change against the default branch
# This might cause conflicts, resolve them
git cherry-pick acda6b84

# Push the cherry pick commit to the remote repository and open a pull request
git push origin backport/43570/stable27

Error: No changes found in backport branch

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Learn more about backports at https://docs.nextcloud.com/server/stable/go.php?to=developer-backports.