net: Remove unused function first_net_device_rcu()

This is unused since commit f04565dd ("dev: use name hash for
dev_seq_ops")

Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250625102155.483570-1-yuehaibing@huawei.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>