Use the generated Voidwalker, remove the hand-written one.

This implies that the code in the repository does not "build
immediately" anymore - some sources must be generated beforehand. For a
better "developer experience", (1) we should ensure that the files are
generated automatically, OR (2) we could deliver the generated files in
the repository.

The usual recommendation is "do *not* deliver generated files". Yet,
people forget that the actual rationale is "because different people may
use different tool versions on different platforms, so the generated
files may vary". So the recommendation holds for files that are
generated by external tools, or whose generation depends on the hosting
platform. Which is *not* the case: the generator is ours, it is
available in the repository, and everybody shall use the same generated
files. So the solution (2) seems OK to me.