AK: Prefer using instead of typedef

Problem:
- `typedef` is a keyword which comes from C and carries with it old
  syntax that is hard to read.
- Creating type aliases with the `using` keyword allows for easier
  future maintenance because it supports template syntax.
- There is inconsistent use of `typedef` vs `using`.

Solution:
- Use `clang-tidy`'s checker called `modernize-use-using` to update
  the syntax to use the newer syntax.
- Remove unused functions to make `clang-tidy` happy.
- This results in consistency within the codebase.

AK/Utf8View.h

@@ -61,7 +61,7 @@ private:
 
 class Utf8View {
 public:
-    typedef Utf8CodepointIterator Iterator;
+    using Iterator = Utf8CodepointIterator;
 
     Utf8View() { }
     explicit Utf8View(const String&);