conncheck: toggle the retransmit flag when pruning pending checks

conncheck: toggle the retransmit flag when pruning pending checks

The function conn_check_update_retransmit_flag() that was introduced to
reenable the retransmit flag on pairs with higher priority than the
nominated one can be merged in priv_prune_pending_checks(), and its
invocation replaced by conn_check_update_check_list_state_for_ready().

The function priv_prune_pending_checks() can also be tweaked to use
the component selected pair priority, instead of getting it from
the checklist. This function is called when at least one nominated pair
exists, so selected_pair is this nominated pair.