Remove useless code

Author: Mygod

mobile/src/main/java/com/github/shadowsocks/MainActivity.kt

@@ -27,7 +27,6 @@ import android.view.KeyCharacterMap
 import android.view.KeyEvent
 import android.view.MenuItem
 import android.view.ViewGroup
-import androidx.activity.addCallback
 import androidx.appcompat.app.AppCompatActivity
 import androidx.browser.customtabs.CustomTabColorSchemeParams
 import androidx.browser.customtabs.CustomTabsIntent
@@ -137,9 +136,6 @@ class MainActivity : AppCompatActivity(), ShadowsocksConnection.Callback, OnPref
         super.onCreate(savedInstanceState)
         WindowCompat.setDecorFitsSystemWindows(window, false)
         setContentView(R.layout.layout_main)
-        onBackPressedDispatcher.addCallback {
-
-        }
         snackbar = findViewById(R.id.snackbar)
         ViewCompat.setOnApplyWindowInsetsListener(snackbar, ListHolderListener)
         stats = findViewById(R.id.stats)