Revert "System themes require Android 10"

This reverts commit 329e98eb14.

app/src/main/java/eu/faircode/email/FragmentOptionsDisplay.java

@@ -24,7 +24,6 @@ import android.content.Context;
 import android.content.SharedPreferences;
 import android.content.res.Configuration;
 import android.graphics.Bitmap;
-import android.os.Build;
 import android.os.Bundle;
 import android.view.LayoutInflater;
 import android.view.Menu;
@@ -36,11 +35,9 @@ import android.widget.AdapterView;
 import android.widget.Button;
 import android.widget.CompoundButton;
 import android.widget.ImageView;
-import android.widget.RadioButton;
 import android.widget.RadioGroup;
 import android.widget.SeekBar;
 import android.widget.Spinner;
-import android.widget.TextView;
 import android.widget.Toast;
 
 import androidx.annotation.NonNull;
@@ -617,13 +614,6 @@ public class FragmentOptionsDisplay extends FragmentBase implements SharedPrefer
         public Dialog onCreateDialog(@Nullable Bundle savedInstanceState) {
             View dview = LayoutInflater.from(getContext()).inflate(R.layout.dialog_theme, null);
             final RadioGroup rgTheme = dview.findViewById(R.id.rgTheme);
-            final RadioButton rbThemeSystem = dview.findViewById(R.id.rbThemeSystem);
-            final RadioButton rbThemeGreySystem = dview.findViewById(R.id.rbThemeGreySystem);
-            final TextView tvSystem = dview.findViewById(R.id.tvSystem);
-
-            rbThemeSystem.setVisibility(Build.VERSION.SDK_INT < Build.VERSION_CODES.Q ? View.GONE : View.VISIBLE);
-            rbThemeGreySystem.setVisibility(Build.VERSION.SDK_INT < Build.VERSION_CODES.Q ? View.GONE : View.VISIBLE);
-            tvSystem.setVisibility(Build.VERSION.SDK_INT < Build.VERSION_CODES.Q ? View.GONE : View.VISIBLE);
 
             final SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(getContext());
             String theme = prefs.getString("theme", "light");