This removes the SequenceChecker that was temporarily used while the rtc::Thread TQ implementation was being fixed. Bug: none Change-Id: Iaa46e47371211ac0a97b2dcaf23cef12b43ee8ea Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/175081 Commit-Queue: Tommi <tommi@webrtc.org> Reviewed-by: Sebastian Jansson <srte@webrtc.org> Cr-Commit-Position: refs/heads/master@{#31256}