net: cleanup init_dummy_netdev_core()

init_dummy_netdev_core() used to cater to net_devices which
did not come from alloc_netdev_mqs(). Since that's no longer
supported remove the init logic which duplicates alloc_netdev_mqs().

While at it rename back to init_dummy_netdev().

Reviewed-by: Joe Damato <jdamato@fastly.com>
Link: https://patch.msgid.link/20250113003456.3904110-2-kuba@kernel.org


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