Change return value of SyncChaptersWithSource.await() (#7715)

* Change return value of `SyncChaptersWithSource.await()`

`updatedToAdd.subtract(reAdded).toList()` never worked as at this point `updatedToAdd` contained ids from db where `reAdded` had default one. Was the same case before the rewrite.

Removed `toDelete` from return value as it was not being used anywhere

* Add doc string

* Use HashSet

Co-authored-by: stevenyomi <95685115+stevenyomi@users.noreply.github.com>

Co-authored-by: stevenyomi <95685115+stevenyomi@users.noreply.github.com>
(cherry picked from commit 11f640cfee)

# Conflicts:
#	app/src/main/java/eu/kanade/tachiyomi/data/library/LibraryUpdateService.kt
#	app/src/main/java/eu/kanade/tachiyomi/ui/manga/MangaPresenter.kt

app/src/main/java/exh/eh/EHentaiUpdateWorker.kt

@@ -209,7 +209,7 @@ class EHentaiUpdateWorker(private val context: Context, workerParams: WorkerPara
             val newChapters = source.getChapterList(manga.toMangaInfo())
                 .map { it.toSChapter() }
 
-            val (new, _) = syncChaptersWithSource.await(newChapters, manga, source)
+            val new = syncChaptersWithSource.await(newChapters, manga, source)
             return new to getChapterByMangaId.await(manga.id)
         } catch (t: Throwable) {
             if (t is EHentai.GalleryNotFoundException) {