du: remove unnecessary implementation of code()

[cakebaker]

The default implementation of code() in the UError trait always returns 1, so our override is redundant.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/pr/bounded-memory (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/tail/retry is no longer failing!
Congrats! The gnu test tests/cut/cut-huge-range is now passing!

[xtqqczze]

There's many other utilities that implement UError::code always returning 1.