[Sent from Firefox] Decommission experiment and related code upon completion

[data-sync-user]

QA Test Details:

We should still see a share_sheet.shared_to event when sharing content, but only with 3 params: activity_identifier, has_share_message, and share_type.

{quote}You can share content via a tab (toolbar share, App Menu > Share), long press context menus on top sites or bookmarks > Share, and on downloaded files or PDFs open in a tab.{quote}

The Sent from Firefox opt-in toggle will no longer appear in Settings. Additional text will no longer be appended to any shares.

┆Issue is synchronized with this Jira Task