The flag was only there to tell whether the {AsyncCompileJob} needs to
be kept alive. We already have this information in all the other fields
of the {AsyncCompileJob}, thus remove it.

R=ahaas@chromium.org

Bug: v8:8238
Change-Id: I8d1d76ba8d622d1816c240e7a824ecf31c3b1ce5
Reviewed-on: https://chromium-review.googlesource.com/c/1297957Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56977}