tipc: simplify resetting and disabling of bearers

Since commit 4b475e3f2f8e4e241de101c8240f1d74d0470494
("tipc: eliminate delayed link deletion at link failover") the extra
boolean parameter "shutting_down" is not any longer needed for the
functions bearer_disable() and tipc_link_delete_list().

Furhermore, the function tipc_link_reset_links(), called from
bearer_reset()  is now unnecessary. We can just as well delete
all the links, as we do in bearer_disable(), and start over with
creating new links.

This commit introduces those changes.

Reviewed-by: Erik Hugne <erik.hugne@ericsson.com>
Reviewed-by: Ying Xue <ying.xue@windriver.com>
Signed-off-by: Jon Maloy <jon.maloy@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>