Remove application_settings_tokens_optional_encryption feature flag

What does this MR do and why?

Remove application_settings_tokens_optional_encryption feature flag

This FF was disabled by default in #27350 (closed) and we didn't see any problems since than.

We planned to remove it in %12.3, but forgot 😞

Other feature flages mentioned in #27350 (comment 215693212) aren't fully migrated, I'll look at them separately.

Related to Remove application_settings_tokens_optional_enc... (#355484 - closed) • Vladimir Shushlin • 16.0

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