Remove SetUseNetworkThreadAsWorkerThread

Support for a worker thread that is distinct from the network thread is
being removed from WebRTC, and the peer connection level test framework
no longer supplies a worker thread of its own. That left
InjectableComponents::worker_thread write only: nothing reads it.

- Remove PeerConfigurer::SetUseNetworkThreadAsWorkerThread().
- Remove InjectableComponents::worker_thread.
- Drop the calls in pc_full_stack_tests, since using the network thread
  as the worker thread is now the only supported behavior.

Bug: webrtc:558821261
Change-Id: Iab6867ca6fee45216a6c06e3cfa0adad8c1abb00
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/502600
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48593}