GitLab

PostAt and ReceiveSends are about to be removed from rtc::Thread,
removing overrides to avoid compile error.

Bug: webrtc:9883
Change-Id: I753fca76b07f5c82b1358f7fa3c4aadd661ecb5c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2000759Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Commit-Queue: Sebastian Jansson <srte@chromium.org>
Cr-Commit-Position: refs/heads/master@{#732805}