tunnel: remove kernel module downloader

Nathan Chance dropped the ball repeatedly and never maintained this in a
consistent way that anybody could use. With Android 12 out now, just
drop it all together. A bummer, but I don't see much of a choice.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

tunnel/src/main/java/com/wireguard/android/backend/WgQuickBackend.java

@@ -53,6 +53,10 @@ public final class WgQuickBackend implements Backend {
         this.toolsInstaller = toolsInstaller;
     }
 
+    public static boolean hasKernelSupport() {
+        return new File("/sys/module/wireguard").exists();
+    }
+
     @Override
     public Set<String> getRunningTunnelNames() {
         final List<String> output = new ArrayList<>();