tsort: remove unreachable error case

[cakebaker]

This PR removes the code to handle the case of zero FILE values. It's not possible to reach that code because we define a default value in clap and so there is always at least one value.

[github-actions]

GNU testsuite comparison:

Note: The gnu test tests/csplit/csplit-heap is now being skipped but was previously passing.
Note: The gnu test tests/pr/bounded-memory is now being skipped but was previously passing.
Note: The gnu test tests/tail/tail-n0f is now being skipped but was previously passing.
Congrats! The gnu test tests/cp/link-heap is now passing!