pimd: Remove mroute when RPF is gone

pimd: Remove mroute when RPF is gone

This does three things:

1) When we get a RPF_FAILURE, remove the mroute associated
with it.
   -> This way when the RPF comes back we can just add the
      mroute in as part of the normal scanning process.
2) When we do a ecmp_nexthop_search return 1 when we found
something we can use.
3) Ignore output from pim_update_rp_nh
   -> When we do a ecmp_nexthop_search ignore the return
      code and do not attempt to gather it up to return
      to the calling function.  It is just ignored
      and we were not taking into account the what of
      multiple RP's we were looking at.

Ticket: CM-17218
Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>