tee: drop cfg for unsupported target

[oech3]

Any other utils don't have such cfg. Just drop it for readability.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/tail/symlink (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/basenc/bounded-memory is now being skipped but was previously passing.
Congrats! The gnu test tests/rm/many-dir-entries-vs-OOM is now passing!
Congrats! The gnu test tests/seq/seq-epipe is now passing!

[cakebaker]

Thanks!