ospfd: Fix ospfd crash in free_nexthop

ospfd: Fix ospfd crash in free_nexthop

Fix ANVL-OSPF-5.1 reported ospfd crash.

vertex_nexthop_free was added as valgrind reported potential
memory leak, but in some cases nexthop would not be available
freed.
The actual nexthop free is part of ospf_canonical_nexthops_free(),
upon trying to free, qfree checks mtype count becomes 0 and asserts.
Removing vertex_nexthop_free() from ospf_spf_flush_parents().

Signed-off-by: Chirag Shah <chirag@cumulusnetworks.com>