Remove strong mode support from rest argument creation.

src/x64/interface-descriptors-x64.cc

Index: src/x64/interface-descriptors-x64.cc
diff --git a/src/x64/interface-descriptors-x64.cc b/src/x64/interface-descriptors-x64.cc
index 1d39d5dedd85099aea97b50c0b4df7d0771b1e0e..79315c70a0ac4de058df1dd43329f3968dfc8189 100644
--- a/src/x64/interface-descriptors-x64.cc
+++ b/src/x64/interface-descriptors-x64.cc
@@ -66,7 +66,6 @@ const Register ArgumentsAccessNewDescriptor::parameter_pointer() { return rdx; }
 const Register RestParamAccessDescriptor::parameter_count() { return rcx; }
 const Register RestParamAccessDescriptor::parameter_pointer() { return rdx; }
 const Register RestParamAccessDescriptor::rest_parameter_index() { return rbx; }
-const Register RestParamAccessDescriptor::language_mode() { return rdi; }
 
 
 const Register ApiGetterDescriptor::function_address() { return r8; }