The negative implication from --single-threaded to --liftoff was
introduced because with --single-threaded, no tier-up to TurboFan is
possible, and TurboFan provides faster code and smaller code

This CL removes this implication. The reason is that this implication
does not only define the default compiler for --single-threaded to
TurboFan, and also completely disables the option to use Liftoff
instead. By removing the implication, and embedder who uses
--single-threaded can decide by themselves if they want to use Liftoff
by setting --liftoff, or if they don't want to use Liftoff by setting
--no-liftoff.

R=clemensb@chromium.org

Bug: v8:11279
Change-Id: Ie6e64a42823b87484135364ecb4589cfd188db5e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2599548Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71917}