ripngd: simplify cleaning up of routing instance

ripngd: simplify cleaning up of routing instance

* Call ripng_clean() only when RIPng is configured, this way we can
  remove one indentation level from this function.
* ripng_redistribute_clean() is only called on shutdown, so there's
  no need to call ripng_redistribute_withdraw() there since the RIPng
  table is already cleaned up elsewhere.
* Deallocate the ripng structure only at the end of the function. This
  prepares the ground for the next commits where all global variables
  will be moved to the ripng structure.

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