Revert of [runtime] Add fast-paths for common conversion methods

src/regexp/regexp-utils.cc

Index: src/regexp/regexp-utils.cc
diff --git a/src/regexp/regexp-utils.cc b/src/regexp/regexp-utils.cc
index d40431866a391388b4e8ebfbbbe75e529e6bb2c5..f6ef076f2c38b781f58d4589e94801ce64130562 100644
--- a/src/regexp/regexp-utils.cc
+++ b/src/regexp/regexp-utils.cc
@@ -174,7 +174,8 @@
 
   ASSIGN_RETURN_ON_EXCEPTION(isolate, last_index_obj,
                              Object::ToLength(isolate, last_index_obj), Object);
-  const int last_index = PositiveNumberToUint32(*last_index_obj);
+
+  const int last_index = Handle<Smi>::cast(last_index_obj)->value();
   const int new_last_index =
       AdvanceStringIndex(isolate, string, last_index, unicode);