If --no-liftoff is passed, we should never compile with Liftoff,
independent of the --wasm-tier-up flag.
The current logic hard-coded Liftoff as baseline tier and TurboFan as
top tier for the tiering case.

Drive-by: Remove unused {default_tier_} field from
          {CompilationUnitBuilder}.

R=ahaas@chromium.org

Bug: chromium:1040061
Change-Id: I65bc5e4aad85396e3b952b41cfe1150f6cd29890
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1992439Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65662}