Parse tasks are not currently used, and will need to be changed significantly
for background compilation, so we remove them for now.

BUG=v8:6093,v8:5203

Change-Id: I44559a94ecca85668f0117629d35aaa5f4075745
Reviewed-on: https://chromium-review.googlesource.com/617140
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47446}