healthcheck: don't advertise med when withdrawing a route

There is no MED in a withdraw message. While exabgp doesn't have a
problem with that, it seems cleaner to not advertise one while this is
just ignored.

Closes: #184
Merge pull request #186 from vincentbernat/fix/healthcheck-no-med-on-withdraw

healthcheck: don't advertise med when withdrawing a route