Remove serialization code for renamed application setting column `asset_proxy_whitelist`

!50824 (merged) renamed the application setting from asset_proxy_whitelist to asset_proxy_allowlist and also removed the serialization code from the old column.

This caused gitlab-com/gl-infra/production#3494 (closed) because before the migration (renaming the column) the Rails code was trying to use load the new (but not yet existing) and fell back to the old (existing but unserialized) column which returned a YAML string instead of any Array. See https://gitlab.com/gitlab-org/gitlab/-/blob/9f4230b0e639dd2659ddb2edc0aaf37c043d2c84/lib/banzai/filter/asset_proxy_filter.rb#L62-63

To mitigate this failure !53272 (merged) brought back the serialization code for the old column which should be removed in the next release.