Remove hidden `--output-format` warning (#22944)
Summary
--
I forgot to include this in #22908, but I think it's okay just to land
this on
`main` instead of the release branch because the warning isn't actually
visible
to users anyway, as reported in #19552. We're stabilizing
`--output-format`
actually being used with `--watch`, so we can just delete this warning.
Fixes #19552
