[util.smartptr.weak] Remove redundant declaration of 'swap' (#4037)

The declaration of the non-member function is already present in the header synopsis [memory.syn] and does not need to be repeated in the class synopsis. We generally don't do this for any other free 'swap' function, either.

Author: JohelEGP

source/utilities.tex

@@ -11050,10 +11050,6 @@
 
   template<class T>
     weak_ptr(shared_ptr<T>) -> weak_ptr<T>;
-
-  // \ref{util.smartptr.weak.spec}, specialized algorithms
-  template<class T>
-    void swap(weak_ptr<T>& a, weak_ptr<T>& b) noexcept;
 }
 \end{codeblock}