Remove memcpy test workaround for name mangling substitutions.

Now that the name mangling is a bit smarter (from commit:
217dc082d5cc2af1cc7c544f51ef15b4abe5be8b), we don't need to
avoid having the same type twice in the function signature.

BUG=none
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/389683003