xargs: don't pretend to support -I.

This was embarrassing... A build script that was using xargs -I silently
started to ignore -I when we switched over to toybox xargs. The owner of
the script has rewritten it to use the shell read builtin instead, but
it's pretty unhelpful to silently ignore an option that radically
changes how xargs behaves. (The -I behavior sounds sufficiently
different from normal behavior that I'm not sure I've understood, and am
just sending this cleanup rather than actually implementing -I correctly.)

Bug: http://b/137832162

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