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/NonnullOwnPtr.h

@@ -45,7 +45,7 @@ class WeakPtr;
 template<typename T>
 class NonnullOwnPtr {
 public:
-    typedef T ElementType;
+    using ElementType = T;
 
     enum AdoptTag { Adopt };