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/NonnullPtrVector.h

@@ -32,8 +32,8 @@ namespace AK {
 
 template<typename PtrType, int inline_capacity = 0>
 class NonnullPtrVector : public Vector<PtrType, inline_capacity> {
-    typedef typename PtrType::ElementType T;
-    typedef Vector<PtrType, inline_capacity> Base;
+    using T = typename PtrType::ElementType;
+    using Base = Vector<PtrType, inline_capacity>;
 
 public:
     NonnullPtrVector()