Remove application_settings_tokens_optional_encryption feature flag
There is currently a usage of application_settings_tokens_optional_encryption
still in the code, but there is no FF yaml and the FF is already requiring encryption by default, so it seems like low-hanging fruit to get rid of it.
Need to ensure that the ApplicationSetting.runners_registration_token
field is encrypted into runners_registration_token_encrypted
.
Context: gitlab-org/quality/triage-reports#6591 (comment 870402034)