webrtcsink: adapt commit "abort stats collection before stopping the Signaller"

Adapt a commit 1 that was introduced as part of the forward port of the MR 'add signal "request-encoded-filter"' 2.

The deadlock said commit was fixing doesn't happen on main branch due to changes in the element design: the Sessions are no longer aborted with the element State held. However, we want to ensure the stats collection task is terminated when the webrtcbin element returns from the Ready to Null transition, meaning that the related resources are released.