mainapp: Removed unused import.

src/main/java/com/owncloud/android/MainApp.java

@@ -27,7 +27,6 @@ import android.app.NotificationChannel;
 import android.app.NotificationManager;
 import android.content.ContentResolver;
 import android.content.Context;
-import android.content.DialogInterface;
 import android.content.Intent;
 import android.content.SharedPreferences;
 import android.content.pm.PackageInfo;