fix unused function in header

Signed-off-by: Maxime CLEMENT <maxime.clement@tier4.jp>

Author: maxime-clem

common/autoware_universe_utils/include/autoware/universe_utils/geometry/gjk_2d.hpp

@@ -19,16 +19,6 @@
 
 namespace autoware::universe_utils::gjk
 {
-
-namespace
-{
-/// @brief get the polygon vertex that is the furthest away from a direction vector
-size_t furthest_vertex_idx(const Polygon2d & poly, const Point2d & direction);
-Point2d support_vertex(const Polygon2d & poly1, const Polygon2d & poly2, const Point2d & direction);
-bool same_direction(const Point2d & p1, const Point2d & p2);
-double dot_product(const Point2d & p1, const Point2d & p2);
-Point2d cross_product(const Point2d & p1, const Point2d & p2, const Point2d & p3);
-}  // namespace
 /**
  * @brief Check if 2 convex polygons intersect using the GJK algorithm
  * @details much faster than boost::geometry::overlaps() but limited to convex polygons