ovn.at: Drop bash specific 'function' keyword.

This keyword is not portable and also optional in bash.
Fixes test on FreeBSD.

CC: Miguel Angel Ajo <majopela@redhat.com>
Fixes: 508b7f9 ("ovn: l3ha, make is_chassis_active aware of gateway_chassis")
Signed-off-by: Ilya Maximets <i.maximets@samsung.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>

tests/ovn.at

@@ -9519,7 +9519,7 @@ for chassis in gw1 gw2 hv1 hv2; do
     ovs-ofctl dump-flows br-int
     echo "--------------------------"
 done
-function bfd_dump() {
+bfd_dump() {
     for chassis in gw1 gw2 hv1 hv2; do
         as $chassis
         echo "------ $chassis dump (BFD)----"