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/RefCounted.h

@@ -64,7 +64,7 @@ class RefCountedBase {
     AK_MAKE_NONMOVABLE(RefCountedBase);
 
 public:
-    typedef unsigned int RefCountType;
+    using RefCountType = unsigned int;
     using AllowOwnPtr = FalseType;
 
     ALWAYS_INLINE void ref() const
@@ -111,16 +111,6 @@ public:
     }
 };
 
-static constexpr bool is_ref_counted(const RefCountedBase*)
-{
-    return true;
-}
-
-static constexpr bool is_ref_counted(...)
-{
-    return false;
-}
-
 }
 
 using AK::RefCounted;