Allow using vector with classes without default ctor in C too

This is similar to be491506a (Java std::vector improvements for types
that do not have a default constructor., 2019-03-01) for Java, except we
don't have to bother with any compatibility constraints for this, not
yet used by anyone. module.

Lib/c/std_vector.i

@@ -24,7 +24,6 @@ namespace std {
         typedef const value_type& const_reference;
 
         vector();
-        vector(size_type n);
         vector(const vector& other);
 
         size_type size() const;