ssh: remove provider => upstart from the Service

Commit 781864dc added provider => upstart to workaround a combination of
a trusty change (init script not shelling out to /lib/init/upstart-job)
and older puppet (which didn't pick the upstart provider by default).

Now we have a newer puppet though, so this shouldn't be needed anymore.

Change-Id: Ic329765c72de406e93d71009c1db2467cde6e8b2