The memory pressure notification logic wasn't correct and given the current users of
the compiler dispatcher aren't posting speculative tasks, it isn't particularly useful.
After removing this, the abort logic can also be simplified significantly by removing the
non-blocking abort logic.

BUG=v8:8041

Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: I584533b58fb717fdca46cc620822914d6bdb28b8
Reviewed-on: https://chromium-review.googlesource.com/c/1278495Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56609}