checkUsernames: send output to stdout, don't include wikiid

checkUsernames: send output to stdout, don't include wikiid

In this case, we're actively looking for invalid usernames, so they're
not errors that should be sent to stderr, they're the expected output.

And wrappers like foreachwiki already include the wikiid so it's
redundant.

Change-Id: I61ea080647b6c327909fff1ba839177d3bc484d8