txn_limbo: simplify txn_limbo_on_parameters_change

There is no need to test @confirm_lsn since we're
continue the list traversing in any case.

I keep "continue" here to be consistent with other "if"s.

Acked-by: Serge Petrenko <sergepetrenko@tarantool.org>
Acked-by: Vladislav Shpilevoy <v.shpilevoy@tarantool.org>
Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>