Make RetireActualLRP remove the route from gorouter

[vlast3k]

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[mariash]

Please see my comment in the issue

cloudfoundry/diego-release#820 (comment)

[vlast3k]

@mariash , i've changed the method, to just send an event, w/o removing the LRP. Indeed i did not recognize that in the other case the Actual LRP is not directly removed.
I also added one additional unit test to validate it