misc: remove ineffective WARN_ON() check from misc_deregister()

misc: remove ineffective WARN_ON() check from misc_deregister()

The WARN_ON(list_empty(&misc->list)) in misc_deregister() does
not catch any practical error conditions:
- For statically allocated miscdevice structures, the list pointers
 are zero-initialized, so list_empty() returns false, not true.
- After list_del(), the pointers are set to LIST_POISON1 and LIST_POISON2,
 so repeated deregistration also fails to trigger the check.

Signed-off-by: Xion Wang <xion.wang@mediatek.com>
Link: https://lore.kernel.org/r/20250827024201.21407-2-xion.wang@mediatek.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>