Simplify thread yielding by removing support for high-priority tasks

Remove the `PostHighPriorityTask` method and the internal
`high_priority_messages_` queue from the `Thread` class.

Instead, change `IsYieldRequested()` to return true whenever there are
any pending tasks in the standard message queue, rather than only
checking for high-priority tasks.

This means that the sequence of batched operations is always executed
when the thread is idle, rather than blocking the thread, ensuring that
media-related calls to the same thread get processed as fast as
possible.

Bug: webrtc:42222804
Change-Id: I758c33dc2c33d79fc5e6beea1926529f78e49255
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/456760
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47249}