net: usb: lan78xx: fix WARN in __netif_napi_del_locked on disconnect

In the Linux kernel, the following vulnerability has been resolved:

net: usb: lan78xx: fix WARN in __netif_napi_del_locked on disconnect

Remove redundant netif_napi_del() call from disconnect path.

A WARN may be triggered in __netif_napi_del_locked() during USB device
disconnect:

WARNING: CPU: 0 PID: 11 at net/core/dev.c:7417 __netif_napi_del_locked+0x2b4/0x350

This happens because netif_napi_del() is called in the disconnect path while
NAPI is still enabled. However, it is not necessary to call netif_napi_del()
explicitly, since unregister_netdev() will handle NAPI teardown automatically
and safely. Removing the redundant call avoids triggering the warning.

Full trace:
lan78xx 1-1:1.0 enu1: Failed to read register index 0x000000c4. ret = -ENODEV
lan78xx 1-1:1.0 enu1: Failed to set MAC down with error -ENODEV
lan78xx 1-1:1.0 enu1: Link is Down
lan78xx 1-1:1.0 enu1: Failed to read register index 0x00000120. ret = -ENODEV
------------[ cut here ]------------
WARNING: CPU: 0 PID: 11 at net/core/dev.c:7417 __netif_napi_del_locked+0x2b4/0x350
Modules linked in: flexcan can_dev fuse
CPU: 0 UID: 0 PID: 11 Comm: kworker/0:1 Not tainted 6.16.0-rc2-00624-ge926949dab03 #9 PREEMPT
Hardware name: SKOV IMX8MP CPU revC - bd500 (DT)
Workqueue: usb_hub_wq hub_event
pstate: 60000005 (nZCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
pc : __netif_napi_del_locked+0x2b4/0x350
lr : __netif_napi_del_locked+0x7c/0x350
sp : ffffffc085b673c0
x29: ffffffc085b673c0 x28: ffffff800b7f2000 x27: ffffff800b7f20d8
x26: ffffff80110bcf58 x25: ffffff80110bd978 x24: 1ffffff0022179eb
x23: ffffff80110bc000 x22: ffffff800b7f5000 x21: ffffff80110bc000
x20: ffffff80110bcf38 x19: ffffff80110bcf28 x18: dfffffc000000000
x17: ffffffc081578940 x16: ffffffc08284cee0 x15: 0000000000000028
x14: 0000000000000006 x13: 0000000000040000 x12: ffffffb0022179e8
x11: 1ffffff0022179e7 x10: ffffffb0022179e7 x9 : dfffffc000000000
x8 : 0000004ffdde8619 x7 : ffffff80110bcf3f x6 : 0000000000000001
x5 : ffffff80110bcf38 x4 : ffffff80110bcf38 x3 : 0000000000000000
x2 : 0000000000000000 x1 : 1ffffff0022179e7 x0 : 0000000000000000
Call trace:
__netif_napi_del_locked+0x2b4/0x350 (P)
lan78xx_disconnect+0xf4/0x360
usb_unbind_interface+0x158/0x718
device_remove+0x100/0x150
device_release_driver_internal+0x308/0x478
device_release_driver+0x1c/0x30
bus_remove_device+0x1a8/0x368
device_del+0x2e0/0x7b0
usb_disable_device+0x244/0x540
usb_disconnect+0x220/0x758
hub_event+0x105c/0x35e0
process_one_work+0x760/0x17b0
worker_thread+0x768/0xce8
kthread+0x3bc/0x690
ret_from_fork+0x10/0x20
irq event stamp: 211604
hardirqs last enabled at (211603): [<ffffffc0828cc9ec>] _raw_spin_unlock_irqrestore+0x84/0x98
hardirqs last disabled at (211604): [<ffffffc0828a9a84>] el1_dbg+0x24/0x80
softirqs last enabled at (211296): [<ffffffc080095f10>] handle_softirqs+0x820/0xbc8
softirqs last disabled at (210993): [<ffffffc080010288>] __do_softirq+0x18/0x20
---[ end trace 0000000000000000 ]---
lan78xx 1-1:1.0 enu1: failed to kill vid 0081/0