exit_sendmail plugin always fails to find koji build ID for scratch builds

[ktdreyer]

scratch builds always hit this code path in atomic_reactor/plugins/exit_sendmail.py:

self.koji_build_id = self.workflow.exit_results.get(KojiImportPlugin.key)
if not self.koji_build_id:
    self.log.info("Failed to fetch koji build ID")

If it's a scratch build, we do not need to try to look up a Koji build ID.

[ktdreyer]

What if we just deleted this plugin entirely: #1710