staging: greybus: loopback.c: remove unused lists

gb_loopback_device::list_op_async is never used except for the
LIST_INIT. The ::list field appears to have a few more uses, but on
closer inspection the linked list of struct gb_loopbacks that it heads
is never used for anything, so there's no reason to maintain it, much
less to keep it sorted.

Reviewed-by: Bryan O'Donoghue <pure.logic@nexus-software.ie>
Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>