irqdomain: Kill of_node_to_fwnode() helper

There is no in-tree users of this helper since b13b41c ("misc:
ti_fpc202: Switch to of_fwnode_handle()"), and is replaced with
of_fwnode_handle().

Get rid of it.

Suggested-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Will Deacon <will@kernel.org>
Link: https://patch.msgid.link/20251020122944.3074811-26-maz@kernel.org

Author: Marc Zyngier

include/linux/irqdomain.h

@@ -730,12 +730,6 @@ static inline void msi_device_domain_free_wired(struct irq_domain *domain, unsig
 }
 #endif
 
-/* Deprecated functions. Will be removed in the merge window */
-static inline struct fwnode_handle *of_node_to_fwnode(struct device_node *node)
-{
-	return node ? &node->fwnode : NULL;
-}
-
 static inline struct irq_domain *irq_domain_add_tree(struct device_node *of_node,
 						     const struct irq_domain_ops *ops,
 						     void *host_data)