zebra: resolve assert when adding ip import-table entry

zebra: resolve assert when adding ip import-table entry

Problem was due to in certain route replace circumstances,
we would mark the old route_entry as removed to delete it but
would leave the selected flag set.  When the rn was pulled off the
work queue for process, we would find both the new re and old re
(being deleted) with the selected flag set and would assert.
In this change, when we decide to delete the old re, we also  mark
it as no longer selected.

Signed-off-by: Don Slice <dslice@cumulusnetworks.com>