{InterpreterCompilationUnit} 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. We can probably also remove
{TurbofanWasmCompilationUnit} in a similar way, which I will do in a
follow-up CL.

R=mstarzinger@chromium.org

Change-Id: I8fc2e18366757573499fd57f909ec8222c27be38
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1587387
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61083}