ospf6d: assorted code cleanup

ospf6d: assorted code cleanup

This commits consists of several changes that positively impact
code reability without introducing any logical change.

Summary of the changes:
* Return earlier in ospf6_abr_range_update() in order to reduce one
  level of indentation;
* Remove ospf6_translated_nssa_originate() since it's nothing other
  than a useless wrapper around ospf6_lsa_translated_nssa_new();
* Change ospf6_abr_translate_nssa() to return void;
* Change ospf6_abr_process_nssa_translates() checking for NSSA areas
  before anything else;
* Remove ospf6_abr_remove_unapproved_translates_apply() since it's a
  small function that is only called in one place;
* Change ospf6_abr_check_translate_nssa() to avoid an LSDB lookup when
  the router isn't an ABR.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>