{LiftoffCompilationUnit} does not store any data, and has a single
method only. Thus remove it, and replace it by a static function.
This saves one field per compilation unit. We can probably also remove
{TurbofanWasmCompilationUnit} and {InterpreterCompilationUnit} in a
similar way, which I will do in follow-up CLs.

R=mstarzinger@chromium.org

Change-Id: I5e1a7d4245fd8bce4862cc83c96f9dac8e0c635e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1587386Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61080}