Remove a redundant argument to xargs.

Remove a redundant argument to xargs.

On GNU, '-I {}' implies '-L 1', i.e., provide one line to each invocation. Since
we are using '-0' (NUL-delimited input) we already implicitly get one item per
line, so the -n 1 is redundant (and also raises a warning).

A simple example: running

    ls --zero | xargs -0 -I {} echo 'aa {}'

prints 'aa ' followed by each item in the current directory - i.e.,
NUL-delimited input really is represented as one line per item.