chore(gui-client): remove incorrect DNS deactivation (#5067)

This call always results in an error because the GUI runs unprivileged
on both Linux and Windows now, so it can't control DNS or deactivate DNS
control. The IPC service has taken over all that, so the GUI not only
shouldn't do it, it can't do it.

<img width="568" alt="image"
src="https://github.com/firezone/firezone/assets/13400041/ad1b861f-4f3e-453a-94b5-d6f21c9eb198">

rust/gui-client/src-tauri/src/client/gui.rs

@@ -268,7 +268,6 @@ pub(crate) fn run(cli: client::Cli) -> Result<(), Error> {
                         Ok(Ok(_)) => 0,
                     };
 
-                    cleanup();
                     tracing::info!(?exit_code);
                     app_handle.exit(exit_code);
                 });
@@ -311,13 +310,6 @@ pub(crate) fn run(cli: client::Cli) -> Result<(), Error> {
     Ok(())
 }
 
-/// Best-effort cleanup of things like DNS control before graceful exit
-fn cleanup() {
-    // Do this redundant deactivation because `Tun` will not automatically Drop before
-    // the process exits
-    connlib_shared::deactivate_dns_control().ok();
-}
-
 /// Runs a smoke test and then asks Controller to exit gracefully
 ///
 /// You can purposely fail this test by deleting the exported zip file during