Revert "Amazing, now I get swap() conflicts from the other side, F.F.S."

The Utility::swap() addition was reverted until a robust solution for
the ambiguity is found.

This reverts commit 3784ca94cf.

src/Magnum/Resource.h

@@ -271,7 +271,7 @@ template<class T, class U> Resource<T, U>::Resource(Resource<T, U>&& other) noex
 }
 
 template<class T, class U> Resource<T, U>& Resource<T, U>::operator=(Resource<T, U>&& other) noexcept {
-    using Utility::swap;
+    using std::swap;
     swap(_manager, other._manager);
     swap(_key, other._key);
     swap(_lastCheck, other._lastCheck);