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/Utf32View.h

@@ -81,7 +81,7 @@ private:
 
 class Utf32View {
 public:
-    typedef Utf32CodepointIterator Iterator;
+    using Iterator = Utf32CodepointIterator;
 
     Utf32View() { }
     Utf32View(const u32* code_points, size_t length)