Principals can be principal/user/<name> or principal/<name> from legacy installs

Signed-off-by: Joas Schilling <coding@schilljs.com>

apps/dav/lib/Migration/RemoveClassifiedEventActivity.php

@@ -126,6 +126,6 @@ class RemoveClassifiedEventActivity implements IRepairStep {
 
 	protected function getPrincipal($principalUri) {
 		$uri = explode('/', $principalUri);
-		return $uri[2];
+		return array_pop($uri);
 	}
 }