Removes job queue flushing in OptimizingCompileDispatcher::Stop when
FLAG_concurrent_recompilation_delay is set. Before this explicit
flushing was run, there was already a wait-loop which ensured the queue
was always empty.

Bug: v8:9810
Change-Id: I620bac9c9d73aead671b178c9450bdd25e6761b1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1934332
Commit-Queue: Dan Elphick <delphick@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Auto-Submit: Dan Elphick <delphick@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65154}