feat(settings): remove the platform mails switch

Broadcasts go to every active user, so the settings switch and its API
call are gone. Admin broadcast texts no longer mention an opt-out.

src/utils/settings.js

@@ -58,11 +58,3 @@ export const markWelcomeSeen = () =>
         method: 'PUT',
         signal: AbortSignal.timeout(REQUEST_TIMEOUT_MS),
     });
-
-/** PUT on Auth, 204: turns the broadcast mails on or off. The caller handles the response status. */
-export const savePlatformMails = (enabled) =>
-    apiFetch(`${import.meta.env.VITE_API_URL}/api/users/settings/platform-mails`, {
-        method: 'PUT',
-        headers: {'Content-Type': 'application/json'},
-        body: JSON.stringify({enabled}),
-    });