Remove unused Nsfw annotation

Extensions now purely declare it through the Gradle config

app/src/main/java/eu/kanade/tachiyomi/annotations/Nsfw.kt

@@ -1,6 +0,0 @@
-package eu.kanade.tachiyomi.annotations
-
-// TODO: remove this when no longer used in extensions
-@Retention(AnnotationRetention.RUNTIME)
-@Target(AnnotationTarget.CLASS)
-annotation class Nsfw