Revert "Introduce a "@box T" type for SIL."

This reverts commit r29474 because it looks like it is breaking the
build of the SpriteKit overlay.

Swift SVN r29482

lib/Serialization/Deserialization.cpp

@@ -3788,14 +3788,6 @@ Type ModuleFile::getType(TypeID TID) {
                                               ->getCanonicalType());
     break;
   }
-
-  case decls_block::SIL_BOX_TYPE: {
-    TypeID boxID;
-
-    decls_block::SILBoxTypeLayout::readRecord(scratch, boxID);
-    typeOrOffset = SILBoxType::get(getType(boxID)->getCanonicalType());
-    break;
-  }
       
   case decls_block::SIL_FUNCTION_TYPE: {
     TypeID interfaceResultID;