Remove redundant m_pos from CJumpNode; add GetRadius() accessor

[Goober5000]

CJumpNode stored a private m_pos field that duplicated the position already held by its Objects[] entry. The field was set once during construction and never updated afterward, so any subsequent movement (e.g. FRED dragging) would leave m_pos stale while Objects[m_objnum].pos held the authoritative value. Every external caller already used GetSCPObject()->pos instead; GetPosition() had zero call sites.

• Remove the m_pos field and its initialization in both constructors, the move constructor, and the move-assignment operator.
• Pass the constructor's position argument directly to obj_create() instead of routing through m_pos.
• Repoint GetPosition() at Objects[m_objnum].pos so it always returns the live, up-to-date position.
• Add GetRadius() to expose the cached m_radius, and use it in jumpnode_get_which_in() instead of the redundant model_get_radius() call.