Remove MSVC default assignment workaround

This seems to work now in VS2015, and its presence causes assumption
failures (static assertions) in dynamic.c++.

c++/src/capnp/layout.h

@@ -627,18 +627,6 @@ private:
 
 // -------------------------------------------------------------------
 
-#if _MSC_VER
-  // TODO(msvc): MSVC insists List{Reader,Builder}::operator= are deleted unless we
-  //   define them explicitly. Don't know why, especially for the readers.
-#define MSVC_DEFAULT_ASSIGNMENT_WORKAROUND(const_, type) \
-  inline type& operator=(const_ type& other) { \
-    memcpy(this, &other, sizeof(*this)); \
-    return *this; \
-  }
-#else
-#define MSVC_DEFAULT_ASSIGNMENT_WORKAROUND(const_, type)
-#endif
-
 class ListBuilder: public kj::DisallowConstCopy {
 public:
   inline explicit ListBuilder(ElementSize elementSize)
@@ -646,8 +634,6 @@ public:
         step(0 * BITS / ELEMENTS), elementSize(elementSize), structDataSize(0 * BITS),
         structPointerCount(0 * POINTERS) {}
 
-  MSVC_DEFAULT_ASSIGNMENT_WORKAROUND(, ListBuilder)
-
   inline word* getLocation() {
     // Get the object's location.
 
@@ -732,8 +718,6 @@ public:
         step(0 * BITS / ELEMENTS), structDataSize(0), structPointerCount(0),
         elementSize(elementSize), nestingLimit(0x7fffffff) {}
 
-  MSVC_DEFAULT_ASSIGNMENT_WORKAROUND(const, ListReader)
-
   inline ElementCount size() const;
   // The number of elements in the list.