Kill [stdlib] attribute.

Now that we have true serialized modules, the standard library can import
the Builtin module without any special direction (beyond -parse-stdlib),
and anyone can include those modules without special direction.

Swift SVN r6752

lib/Serialization/SerializedModuleLoader.cpp

@@ -119,8 +119,7 @@ static Module *makeTU(ASTContext &ctx, AccessPathElem moduleID,
 
 
 Module *SerializedModuleLoader::loadModule(SourceLoc importLoc,
-                                           Module::AccessPathTy path,
-                                           bool isStdlibImport) {
+                                           Module::AccessPathTy path) {
 
   // FIXME: Swift submodules?
   if (path.size() > 1)