Since multisource extensions are no longer a thing, we now simply rely on the flag at the extension level, i.e. the per-Source/SourceFactory `@Nsfw` annotation is no longer checked.
We'll have to remove all of the annotation usages from the existing sources, which will also effectively break the setting for older versions of the app.
(cherry picked from commit a2cf210a524894e5c1d1f062e4813086b46777a4)
# Conflicts:
# app/src/main/java/eu/kanade/tachiyomi/ui/browse/extension/ExtensionHolder.kt
# app/src/main/java/eu/kanade/tachiyomi/ui/browse/extension/ExtensionPresenter.kt
