scheduler: Remove unused REVERSE_LOCK

Summary:
> No longer needed after [[bitcoin/bitcoin#17270 | core#17270]]

This is a backport of [[bitcoin/bitcoin#19090 | core#19090]] [1/6]
bitcoin/bitcoin@fa609c4

Backport note: the original PR has 7 commits, but the last one is not relevant to our codebase.

Test Plan:
With tsan:
`ninja check`

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9955

src/scheduler.cpp

@@ -26,9 +26,6 @@ void CScheduler::serviceQueue() {
     // waiting or when the user's function is called.
     while (!shouldStop()) {
         try {
-            if (!shouldStop() && taskQueue.empty()) {
-                REVERSE_LOCK(lock);
-            }
             while (!shouldStop() && taskQueue.empty()) {
                 // Wait until there is something to do.
                 newTaskScheduled.wait(lock);