This removes the support to serialize copies of {CodeStub} codes during
native module serialization. It is still possible to serialize builtins
and all code objects copied from the GC heap are builtins by now.

R=ahaas@chromium.org

Change-Id: If009a82a9d7c7080f70f344040ebb91f20b8cc1a
Reviewed-on: https://chromium-review.googlesource.com/1012081
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52628}