util/StringView: remove methods SetEmpty(), Empty()

src/tag/Builder.cxx

@@ -230,7 +230,7 @@ void
 TagBuilder::AddEmptyItem(TagType type)
 {
 	tag_pool_lock.lock();
-	auto i = tag_pool_get_item(type, StringView::Empty());
+	auto i = tag_pool_get_item(type, "");
 	tag_pool_lock.unlock();
 
 	items.push_back(i);

src/util/StringView.hxx

@@ -49,15 +49,6 @@ struct StringView : ConstBuffer<char> {
 	constexpr StringView(std::nullptr_t n) noexcept
 		:ConstBuffer(n) {}
 
-	static constexpr StringView Empty() noexcept {
-		return StringView("", size_t(0));
-	}
-
-	void SetEmpty() noexcept {
-		data = "";
-		size = 0;
-	}
-
 	gcc_pure
 	pointer_type Find(value_type ch) const noexcept {
 		return StringFind(data, ch, size);