zebra: Allow blackhole route deletion for prefixes

zebra: Allow blackhole route deletion for prefixes

With the recent change to just pass the prefix in
for the RTM_DELROUTE, for blackhole routes we
had stopped modifying the req.rtm_type to
be the appropriate type for blackhole routes.

Since we are just deleting on the route, and
zebra is never going to really install the same
route multiple times then we do not need
to specify the req.r.rtm_type for the deletion
command.

Ticket: CM-20616
Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>