增加纯黑暗色模式，清理代码

app/src/main/java/io/legato/kazusa/constant/PreferKey.kt

@@ -191,7 +191,7 @@ object PreferKey {
     const val mangaClickActionBL = "mangaClickActionBottomLeft"
     const val mangaClickActionBC = "mangaClickActionBottomCenter"
     const val mangaClickActionBR = "mangaClickActionBottomRight"
-    const val isFirstLaunch = "isFirstLaunch"
+
     const val permissionChecked = "permissionChecked"
     const val notificationsPost = "notificationsPost"
     const val ignoreBatteryPermission = "ignoreBatteryPermission"