zebra: fix imported static routes deletion

zebra: fix imported static routes deletion

When unconfiguring 'no import <table>', a static route imported
from a routing table number is never deleted.

When importing a route from a given table, a default distance of
15 is applied. At the time of deletion, when trying to compare
the original route with the new one, the distance does not match,
because the static route applies a default distance of 1.

If the imported route has the distance set, unset the distance
flag to avoid comparing it.

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>