Clean up preference extensions/items a bit

app/src/main/java/eu/kanade/tachiyomi/util/preference/PreferenceExtensions.kt

@@ -1,16 +0,0 @@
-package eu.kanade.tachiyomi.util.preference
-
-import tachiyomi.core.preference.Preference
-
-operator fun <T> Preference<Set<T>>.plusAssign(item: T) {
-    set(get() + item)
-}
-
-operator fun <T> Preference<Set<T>>.minusAssign(item: T) {
-    set(get() - item)
-}
-
-fun Preference<Boolean>.toggle(): Boolean {
-    set(!get())
-    return get()
-}