Remove unused method

fix #2410

include/boost/beast/core/impl/multi_buffer.hpp

@@ -1200,15 +1200,6 @@ destroy(list_type& list) noexcept
         destroy(*it++);
 }
 
-template<class Allocator>
-void
-basic_multi_buffer<Allocator>::
-destroy(const_iter it)
-{
-    auto& e = list_.erase(it);
-    destroy(e);
-}
-
 template<class Allocator>
 void
 basic_multi_buffer<Allocator>::