Clear old item groups when they are overridden. (#8753)

This fixes overridden items keeping their old groups in the group to
items mapping even after their groups have been changed in lua.
It also prevents a more widespread issue where overriding an item
will add its content ID *twice* to the mapping, resulting in odd
behaviour in features such as ABMs.

src/nodedef.h

@@ -668,6 +668,14 @@ private:
 	 */
 	void addNameIdMapping(content_t i, std::string name);
 
+	/*!
+	 * Removes a content ID from all groups.
+	 * Erases content IDs from vectors in \ref m_group_to_items and
+	 * removes empty vectors.
+	 * @param id Content ID
+	 */
+	void eraseIdFromGroups(content_t id);
+
 	/*!
 	 * Recalculates m_selection_box_int_union based on
 	 * m_selection_box_union.