Clean up download ahead logic

- Remove redundant chapter sorting logic when fetching next chapter(s)
- Remove redundant download queue checks (it'll handle already queued or downloaded items)
- Trigger download ahead when read >= 25% of chapter rather than 20%
- Rely on download cache when checking if next chapter is downloaded to avoid jank (fixes #8328)

app/src/main/java/eu/kanade/tachiyomi/util/system/BooleanExtensions.kt

@@ -1,5 +1,3 @@
 package eu.kanade.tachiyomi.util.system
 
-fun Boolean.toInt() = if (this) 1 else 0
-
 fun Boolean.toLong() = if (this) 1L else 0L