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/RefPtr.h

@@ -128,7 +128,7 @@ struct RefPtrTraits {
 
     static constexpr FlatPtr default_null_value = 0;
 
-    typedef std::nullptr_t NullType;
+    using NullType = std::nullptr_t;
 };
 
 template<typename T, typename PtrTraits>