Tell irrlicht if we handle a key or not.

We can remove the function in MtNativeActivity now
as it serves precisely that purpose: to tell irrlicht
that we handled the esc key.

TODO for later:
 * Perhaps try to find a more performant container than KeyList

build/android/src/main/java/net.minetest.minetest/MtNativeActivity.java

@@ -19,11 +19,6 @@ public class MtNativeActivity extends NativeActivity {
 	public void onDestroy() {
 		super.onDestroy();
 	}
-	
-	@Override
-	public void onBackPressed() {
-	}
-
 
 	public void copyAssets() {
 		Intent intent = new Intent(this, MinetestAssetCopy.class);