{TurbofanWasmCompilationUnit} does not store any data except for a
pointer back to the {WasmCompilationUnit}, and has a single method only.
Thus remove it, and replace it by a static function.
This saves one field per compilation unit.

R=mstarzinger@chromium.org

Change-Id: I2bcb9246c65e6971aa747488ea631886ca3bc037
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1587388
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61084}