VM: Remove unnecessary _Num class from runtime/lib.

runtime/lib/double.cc

Index: runtime/lib/double.cc
diff --git a/runtime/lib/double.cc b/runtime/lib/double.cc
index dbeb44705b81d380aa62b8bcd9b55b976e3ca04e..2706269ad72e79bdb4f278f8f29c88640848e5fa 100644
--- a/runtime/lib/double.cc
+++ b/runtime/lib/double.cc
@@ -230,6 +230,14 @@ DEFINE_NATIVE_ENTRY(Double_parse, 3) {
 }
 
 
+DEFINE_NATIVE_ENTRY(Double_toString, 1) {
+  const Number& number = Number::CheckedHandle(arguments->NativeArgAt(0));
+  Heap::Space space = isolate->heap()->ShouldPretenure(kOneByteStringCid) ?
+      Heap::kPretenured : Heap::kNew;
+  return number.ToString(space);
+}
+
+
 DEFINE_NATIVE_ENTRY(Double_toStringAsFixed, 2) {
   // The boundaries are exclusive.
   static const double kLowerBoundary = -1e21;