Remove some unreachable code.

Fixes #175. This was broken when throwing an exception on missing
positional parameters in #169.

include/cxxopts.hpp

@@ -1663,7 +1663,6 @@ ParseResult::consume_positional(std::string a)
     {
       throw option_not_exists_exception(*m_next_positional);
     }
-    ++m_next_positional;
   }
 
   return false;