OAPI: remove shutdown worker state

Remove the `shutdown` worker state. It was only used in Flamenco 2 to
signal to the worker that it should go offline. In other words, it didn't
use the target state in the request.

As of this change, just send `offline` as the requested state for the
worker if you want it to go offline.

pkg/api/flamenco-openapi.yaml

@@ -777,7 +777,7 @@ components:
 
     WorkerStatus:
       type: string
-      enum: [starting, awake, asleep, error, shutdown, testing, offline]
+      enum: [starting, awake, asleep, error, testing, offline]
 
     WorkerSignOn:
       type: object