After fixing https://crbug.com/v8/8916, background compilation scales
far beyond 10 threads, especially for TurboFan (where much more work is
parallelizable). Thus, remove the limit of 10 background compilation
tasks, and use all available threads instead.

R=mstarzinger@chromium.org

Bug: v8:8916
Change-Id: I13c30777e3c85b2de7901b5eac3e6a41457a56f9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1893348Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64724}