OAPI: remove last_activity field from RegisteredWorker

A task's "activity" field is a string that describes the last-performed
operation on the task. In Flamenco 2, a Worker's "last activity" is the
timestamp at which it was last seen. Using the same term for different
things is confusing, and thus gets removed until the Worker's "last seen"
functionality is reimplemented.

pkg/api/flamenco-openapi.yaml

@@ -716,7 +716,6 @@ components:
         address: { type: string }
         status: { $ref: "#/components/schemas/WorkerStatus" }
         platform: { type: string }
-        last_activity: { type: string }
         software: { type: string }
         supported_task_types:
           type: array
@@ -728,7 +727,6 @@ components:
         - status
         - platform
         - current_task
-        - last_activity
         - software
         - supported_task_types