NodePath: Remove unimplemented `get_parent()` method

Fixes #48100.

(cherry picked from commit 12f826da7d8fca9d365ec5b737fdcd891a72b7ce)

core/node_path.h

@@ -77,8 +77,6 @@ public:
 
 	void prepend_period();
 
-	NodePath get_parent() const;
-
 	_FORCE_INLINE_ uint32_t hash() const {
 		if (!data)
 			return 0;