Fix Action Text sanitizer initialization

[flavorjones]

Motivation / Background

Using initializer in the engine may run the block before application initializers. Instead, use config.after_initialize to ensure that application initializers take effect properly.

Also, don't bother deleting the config value, since that pattern isn't needed here (as it is in other railties like action_view/railtie.rb).

See similar fix for Action View in #48747.

Detail

Replace initializer with config.after_initialize.

Additional information

I'm not sure how to test this behavior properly other than to set up an application and run the code. I would love to learn how to better test initializer order.

Checklist

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• This Pull Request is related to one change. Changes that are unrelated should be opened in separate PRs.
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• Tests are added or updated if you fix a bug or add a feature.
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