media: v4l: async: Fix duplicated list deletion

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

media: v4l: async: Fix duplicated list deletion

The list deletion call dropped here is already called from the helper function in the line before. Having a second list_del() call results in either a warning (with CONFIG_DEBUG_LIST=y):

list_del corruption, c46c8198->next is LIST_POISON1 (00000100)

If CONFIG_DEBUG_LIST is disabled the operation results in a kernel error due to NULL pointer dereference.