[sgen] Disable concurrent queue redirection

[sgen] Disable concurrent queue redirection

We used to support the case where workers are running and when the main gc thread populates the redirected gray queue, these queue sections are moved to the workers distribute gray queue from where they are distributed to all the workers. In order to simplify the process, the main gc thread should never do any relevant gc work concurrently with the workers. This means that we no longer need to handle the above case.