sparse set: drop redundant access specifier

src/entt/entity/sparse_set.hpp

@@ -275,7 +275,6 @@ protected:
         packed[pos] = traits_type::combine(static_cast<typename traits_type::entity_type>(std::exchange(head, pos)), tombstone);
     }
 
-protected:
     /**
      * @brief Erases entities from a sparse set.
      * @param first An iterator to the first element of the range of entities.