Removes FF service_desk_new_note_email_native_attachments

What does this MR do and why?

Contributes to [Feature flag] Rollout of `service_desk_new_not... (#386860 - closed)

After global rollout and enabling the FF by default this MR aims to remove all Feature Flag related code for the FF service_desk_new_note_email_native_attachments.

The feature sends attachments as native email attachments in Service Desk emails. It displays a warning message in the editor (when attachments are in the active editor) that GitLabs sends these as native attachments, but only up to a cumulative size of 10MBs.

• Create a merge request to remove <feature-flag-name> feature flag. Ask for review and merge it.
  • Remove all references to the feature flag from the codebase.
  • Remove the YAML definitions for the feature from the repository.
  • Create a changelog entry.

MR acceptance checklist

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