ripd: simplify cleaning up of routing instance

ripd: simplify cleaning up of routing instance

* Call rip_clean() only when RIP is configured, this way we can
  remove one indentation level from this function.
* rip_redistribute_clean() is only called on shutdown, so there's
  no need to call rip_redistribute_withdraw() there since the RIP
  table is already cleaned up elsewhere.
* There's no need to clean up the "rip->neighbor" nodes manually before
  calling route_table_finish().
* Deallocate the rip 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 rip structure.

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